recent-view: Verify a path exists before adding an icon

Fixes warnings when switching from the recent view to
a document back and forth.
recent-view: Use a GtkIconView and get rid of most of libgd cut-n-paste
recent-view: Allow to open documents from recent view with the keyboard
recent-view: Do not sort the GtkTreeModel

We already sort the recent items ourselves and everytime the recent
files change we clear and fill the model again.