fix the alternate selection modes (shift + alt + ctrl)

[a.l.e]

this is a continuation of a thread started in

http://forums.scribus.net/index.php/topic,3084.msg14923.html#msg14923

summary:

- ale: a greedy selection mode has been added recently (press the alt key).
- gibbz: shift select should invert selection, which it does when clicking a individual object. But not when holding shift + selecting a region

[a.l.e]

@gibbz : is this related to https://bugs.scribus.net/view.php?id=15554 ?

[Gibbz]

Yep it is

Ive also done another for adding shift to the resize gesture:
https://bugs.scribus.net/view.php?id=15563