Add mobile layout

Add a mobile specific layout (QT_QUICK_CONTROLS_MOBILE=1) so that Elisa can be used on Plasma Mobile.

• Add mobile "now playing" footer panel (replacing the desktop header) (TODO: add volume controls)
• Fix touch scrolling
• Port playlist to mobile format
• Expandable sidebar to view icon labels
• Rework track list items so that a single touch would start playing a song
• Rework track list items so that all of the context elements are in an overlay sheet (on desktop, they show on hover)
• Move header elements to a second bar on mobile (except for search)

Track list:

Player:

Sidebar:

Playlist queue:

Track options:

Metadata editor:

Settings: