Fixed notebook tab switching by keyboard shortcut

When you choose to hide any of the tabs in the various UI components, such as unchecking "Preferences->Interface->Sidebar->Show documents list" or unchecking the various "_visible" preferences in Preferences->Various, the hidden tab prevents the keybindings for "Switch to left document" and "Switch to right document".

This fixes the bug so that it ignores non-visible tabs and still switches with wrapping.

Tested on: 
geany 1.23 (git >= db8e198) (built on Feb 20 2013 with GTK 2.24.4, GLib 2.28.6)