Add VetoableListSelectionModel.

Using this, don't allow the user to deselect things in the alias
manager dialog.

Change-Id: I47b086c630d7992bd8dd659aac832ebce9feefb6
Reviewed-on: http://gerrit.dmdirc.com/3542
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>
If deleting selected alias, nullify selected alias.

Change-Id: I53d173809d1d23f937be289dc533de17477c4e50
Reviewed-on: http://gerrit.dmdirc.com/3543
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>