Suggestion : make #@gui parsing less strict

[dtschump]

When defining a filter parameter for the G'MIC-Qt plug-in, the following line is not accepted:

#@gui: Alpha = float(0,0,100)

We need an extra-space between #@gui and :, i.e.

#@gui : Alpha = float(0,0,100)

Would it be possible that the #@gui parser accepts also #@gui:?
(applies also for localized filters as #@gui_fr:).

[c-koi]

Achieved by f63c614.