[NbtkGrid] Set grid reactive when initialising
[NbtkGrid] Set grid reactive when initialising

Otherwise the grid does not receive scroll events, preventing the
scroll-wheel support from working.

Fixes: MB:#1894

clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx