Major Refactor

Large refactor of all CSS rules, still in testing
- Several quick hacks now use generic QT solutions (tabs, scrollbar,
treeview/listview borders)
- Elements are now separated by QT-generic and Mumble-specific
- Scrollbars have borders
- Sliders now use a fill color and rounded corners
- Toolbar icons now have nicer background color and borders
- Lists and treeviews are now more condensed
- Minor color and icon tweaking
- Minor dialogs improved (color picker, calendar widget, font picker,
about window)
- Changing $base-padding now calculates sizes correctly across the theme
- Messing around with applying border radius across the theme (see
$base-border-radius in source folder)