Channel strip UI layout and code improvements (#68)

Channel strip UI layout and code improvements (#68)

* Overhaul channel fader/meter layout:
  * Put value readout/edit entry and peak value readout next to each other
    above the fader and meter. When channels are shrunk, they are displayed
    one under the other.
  * Add tick marks for left/center/right on balance slider
    and add tooltip displaying left/right value. This also
    changes the appearance of slider handle depending on theme.
  * Convert occurences of `Gtk.HBox` and `Gtk.VBox` to `Gtk.Box`.
  * Remove `Gtk.Frame`s from layout and set border style via CSS where needed.
* You can now shrink the width of input *and* output channels and it also
  reduces the width of the meter display.
* On project load, give input focus to fader of last added channel and deselect
  volume entry widget.
* Some code reorganization and cleanup in `abspeak`, `channel`, and `meter`
  module:
  * Refactor channel strip UI creation to reduce DRY.
  * Use `super()` instead of directly accessing the super-class where
    appropriate
  * Unify `widen` and `narrow` `Channel` methods.
  * Remove outdated size request handling methods in `MeterWidget`.
  * Style context getter for `AbspeakWidget` returns context for wrapped label
    widget.

Fixes: #67
Signed-off-by: Christopher Arndt <chris@chrisarndt.de>