PanedCollapsible: initial implementation of the Paned container with a collapsible child.
PanedCollapsible: fixed typo and save expander as object variable
PanedCollapsible: set options to the current expander
widget: updated the PanedCollapsible and replaced the GtkPaned on the interface. Now it supports the basic expand/collapse functionality and blocking the handle when collapsed
Finished porting some stuff to the CollapsiblePaned widget and removed now obsolete code from coverart_browser_source
Added some comments and rewrote a function to reuse a portion of code instead of repeating it
widgets: added some more comments to the CollapsiblePaned widget
Merge branch 'collapsible-paned'

Refactored code that managed the paned behavior of the source main view on it's
own widget, which also takes care of the expandable on one of it's children.
This new widget abstracts the storing of the paned handle position, handles the
expandable child and handles the workaround to keep the handle in a fixed place
(when the expandable child is collapsed).