easer variable selection in OMEdit

[massimo ceraolo]

In OMEdit r17838 to select a variable to be plotted one has to click exactly inside the corresponding checkbox.
In a lot of other programs, instead, to select a check-box item it is sufficient to click on the corresponding name. This is also the default behavior in Qt (at least Qt 5.1), and AFAIK OMEdit is built using Qt.

I think that this correction should be easy to implement, and useful for the final user.

[Adeel Asghar]

Done in r18218.