open-document selector: replace headerbar file controls

The open/recent menu controls in headerbar let his place to a 'Open' button
with a popover dialog containing a search entry accepting wildcards,
a recent file list and a 'Other Documents' button for loading other files..

GeditOpenDocumentSelector's widget use the recentchooser interface
but without icons and tooltips.

As before, you can change the max-recents setting.
( gsettings set org.gnome.gedit.preferences.ui max-recents 'value' )