Re-organize CSS styling and set distinctive colors for active toggle buttons (#90)
* Move all CSS styles and styling related functions into a new `styling` module.
* Provide helper function to add CSS provider in this module.
* Load default CSS in `JackMixer.create_ui()` using the above function (user CSS file loading will probably be added later).
* Use CSS classes instead of widget names for channel strip elements (buttons etc.).
* Set background color of toggle buttons in channel strips when hovered or active to more distinct colors.
Signed-off-by: Christopher Arndt <chris@chrisarndt.de>