[Mousepad 0.5] Disable "Select All" keybinding to prevent conflicts

In !42 (merged), a keybinding was added for the "Select All" action, which has a side effect: trying to select all the text in the search bar entry actually selects the text in the view (I guess it would be the same for any other child widget with a GtkEntry in a MousepadWindow).

This MR deactivates this keybinding at the action level (not at the widget level), while preserving its mention in the menubar.