Minor changes
- Improve headerbar buttons on hover state
- Improve budgie panel appearance
- KDE: Add sddm theme
- KDE: Add sddm theme to dependencies
- Improve disabled buttons style
- KDE: Improve splashscreen
- KDE: Add solid variant to kvantum
- Gnome-shell: Add visual separation to osd-panel related to #69
- Gnome-shell: Remove unnecessary shadow in some widgets