Add styling for QScrollBar/QComboBox controls for the Steam Deck theme

[loathingKernel]

Adds theming for QScrollBar/QComboBox controls for the Steam Deck theme

Preview:

The icon file paths are injected in to the CSS string dynamically right now. I would like to move them into a QResources file but I wanted to get the ok from you before going forward with that. The QResources as far as the theme is concerned should include the QSS file along with any images it uses. The icons are modified from Font Awesome icons and I should check up on their exact license.

Generally I consider this WIP that should take some time due to availability. Feel free to comment with any concerns in the meantime though.

Edit: Regarding the license, it should be ok to use. These icons are take from the FontAwesome repository, they are caret-down and caret-up