keyboard: add a reset all button

As described in the proposed mockups [1], the Keyboard panel
should have a Reset All button above the list of shortcuts that
allows the user to quickly reset all the shortcuts to their
default keybinding. The current implementation, however, lacks
this button.

Fix that by adding a "Reset All" button, and implementing the
reset all action. A message dialog is shown in order to confirm
the action, and custom shortcuts are not reset (unless the conflict
with the default keybinding of another standard shortcut).

[1] https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/system-settings/keyboard/keyboard-wires.png

https://bugzilla.gnome.org/show_bug.cgi?id=777840