keyboard: Add 'reverse' helpers to CcKeyboardItem

In order to handle shortcuts which can be reversed (for example,
super-space and shift-super-space to switch input methods
forward/backward), we are going to add new attributes to the xml files
describing the keyboard shortcuts to show in the panel.

This commit is a first step towards that and adds the notion of
'reverse' items to CcKeyboardItem.

We will then indicate in the xml description files that
'switch-input-source' is reversed by 'switch-input-source-backward' and
that 'switch-input-source-backward' reverses 'switch-input-source'.

https://bugzilla.gnome.org/show_bug.cgi?id=731618