Add a PraghaToolbar struct to clean cwin.
TODO: Really clean!.
Move albumart destruction to PraghaToolbar
Change names and arguments that depenend of PraghaBackend.
Rename album_art_frame_press_callback to pragha_toolbar_album_art_activated.
Port album-art-size to PraghaPreferences.
Add pragha-toolbar.h, and move some function prototypes here.
Also rename some functions, a declare static others..
Merge branch 'CleanToolbar'