Complete overhaul of how widgets are handled in Trumpet.

* `main.ui` has been broken into different components
* Each major component has a corresponding .rs file in `widgets`
module
* Composite widgets use their children as struct fields,
with the "container" being public
* The `ApplicationWindow` is not created with `gtk::Builder`,
but in `app.rs` and major widgets are added to it.
* Steps have been taken toward multi-threading with spawning a
thread to authorize the application
* The `connect` module is gone, and callbacks are set up within
the individual widgets' creation process.
* New view switcher patterns have been implemented for both
desktop and mobile interface.