Previously, there were multiple redundant lists of possible values for
this setting. This uses one struct to define not only all the possible
values, but also anything specific to the alternate characters key, so
this can be used for other modifiers.
This also changes the style of the dialog to use a GtkListBox.
This adds a widget called `CcKeyboardShortcutRow`, sub-classing
`GtkListBoxRow`, to handle a shortcut row. This makes the implementation
a bit tidier, rather than handling it all in `CcKeyboardPanel`, and
allows the widgets that compose the row to be laid out in xml.
This is a prerequisite for moving the shortcuts to a new dialog.
The current CcKeyboardPanel used to manage keyboard
editing through GtkCellRendererAccel, which was replaced
when we moved to use a GtkListBox. Because of that,
the ability to edit shortcuts is now missing.
Re-add shortcut editing capabilities through a new dialog,
which is done according to the latest mockups available.
https://bugzilla.gnome.org/show_bug.cgi?id=769063
