GtkCellRendererAccel: Try harder to capture bound keys

Add a grab on the editable widget, so that we can capture
also shortcuts which are bound in the application itself.

https://bugzilla.gnome.org/736203
Avoid compiler warnings

The previous commit was sloppy with parameter types.
Don't forget to allocate popovers

When GtkApplicationWindow has a menubar, we don't chain up
to the GtkWindow size_allocate, which used to position the
popovers. Move that to _gtk_window_set_allocation() which
is always called by GtkApplicationWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=736205
inspector: Don't unref NULL objects

When closing the application window before closing the inspector window,
the selected widget will be set to NULL once and all the property pages
will try to g_object_weak_unref that NULL widget, so bail out in
gtk_inspector_prop_list_set_object if it's NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=736201
Better thumbnail for GtkTreeView

This one makes it more obvious that it is a tree with multiple
columns and not just multiline text.

https://bugzilla.gnome.org/show_bug.cgi?id=528800
Updated Latvian translation