This PR adds the ability to control whether the two system bars (status bar up top and navigation bar at the bottom) are visible and the colours/transparency they should have.

For now I simply made the status bar transparent so as to remove the blue stripe we had at the top, but I also added and commented out the code to change the navigation bar and show/hide either of them, just so we have the option and it's easy to change. It's a surprisingly simple little composable:

@Composable
internal fun SystemBars() {
    rememberSystemUiController().apply {
        setStatusBarColor(
            color = Color.Transparent,
            darkIcons = true
        )
        // setNavigationBarColor(
        //     color = Color.Transparent,
        //     darkIcons = true
        // )
        // this.isNavigationBarVisible = false
        // this.isStatusBarVisible = false
    }
}

I really tried to keep this PR from getting too big but clearly it got away from me. Not only that, but I also had to do a rebase on commits I had not brought in when I started; I'm sorry if this is a bit of a pain to review.

Overall, I aimed to keep the UI very simple and mostly just bring the structure required for further changes. My main idea was to create a bottom navbar with two core screens: a wallet/balance screen to eventually make transactions, and a settings screen with all of the options and extra things that a user needs to do. Each of those options/settings is a button that leads you to a dedicated screen for that feature (opening a channel, connecting to a peer, etc.)

Here is basically what it brings in:

Here are the cliff notes:

1. The onchain wallet is now being created/restored automatically upon startup in the DispatchActivity. If the file with the mnemonic exists it uses it and restores the wallet, otherwise it creates a new mnemonic and uses that.
2. The bottom bar works the following way: I have a container screen called HomeScreen, which contains a bottom nav bar and a sort empty container space above it, in which we place the other screens. The two screens available from the navbar are the Wallet and the Settings. From the settings screen, you can launch into different small sort of "sub-screens" (some of those could be turned into bottom sheets at some point too). The interesting file to take a look at is Navigation.kt. This is the core of the new navigation system using the Compose libraries.
3. I removed a few unused imports, added 3 icons.

I'm wondering if you'd like to add a few extra screens with maybe a bottom nav bar to split up the features/buttons into separate locations or if maybe you're trying to keep this as simple as it gets on the UI side just to focus on the LDK part of the wallet.

Let me know, I can take a look and maybe PR a bit of navigation/screens if that's something you'd like to add.

This adds a screen to list all the usable channels and the available balance.

The logic for the balance/progress bar is not quite right (will update)

I initially had written a few of the logging lines in a more traditional way with a specific tag for each file/class, but I'm now changing my tune; I think for now it makes sense to have more like a global tag to start, and maybe customize if or when it's required for specific logging entries.

This PR simply creates a global default tag I called LDKTAG (not sure about the name but I needed something), and switches all log entries in the current app to use that tag. This makes it very easy to follow all tags emitted by us. I also fixed a small bug I introduced in the previous refactoring (capitalization of the getLatestBlockHash() method).

This PR adds the OnchainWallet object which groups together most of the tasks related to onchain stuff.

Calls throughout the app now become OnchainWallet.sync(), OnchainWallet.getNewAddress(), or OnchainWallet.buildFundingTx().

Also worthy of review is the fact that I wasn't sure if the Global object really needed to be extending the Android Application class. I refactored it to be just a singleton, and it appears to be working fine on my end. Let me know if you'd prefer to keep it as it was.