ishell: Make the about dialog non modal
shell: Update website and copyright years in about dialog
shell: Move about dialog to the app menu and make it independent from the document
shell: Use a GtkHeaderBar instead of a GtkToolbar and window decorations

Based on patch by Yosef Or Boczko.

https://bugzilla.gnome.org/show_bug.cgi?id=711078
Revert "shell: Use a GtkHeaderBar instead of a GtkToolbar and window decorations"

This reverts commit c9127524e1409d582f038439e9143695d1fadc0d.

It broke the fullscreen mode.
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.