Add error prevention validations

[RedBearAK]

@nclarius

There were frequent TypeError messages in the journal showing up, and I believe they were contributing to inconsistency with the script being able to maintain the app groupings. Sometimes elements that were being checked were "undefined". The additional validations should prevent those errors, and seem to make the overall functioning of the app grouping much more consistent. Both Alt+Tabbing and clicking on a background window in a group will successfully bring all the related windows forward in my testing.

Prior to adding this it seemed to be possible to damage the window list of an app group by clicking on a "background" window of a "background" app group, leading to only one window of that app group coming forward from then on. I was able to trigger this in two different app groups.

After adding these validation checks, I can't seem to replicate the issue. The grouping works much better.

[RedBearAK]

@nclarius - This PR should close issue #6 when merged.

[RedBearAK]

Closing this in favor of PR #8 which also includes these validation fixes in addition to merged support for KDE 5/6.