Inhibit device falling asleep while call is active, fix to #56

Please note, I am not a programmer and I barely understood what I have done here. As described in #56 (closed) , I took some lines of code from !31 (merged) until I was able to build a Debian package to test this. Quite surprisingly, test was successful and PP stayed awake during the call regardless of suspension timeout being reached!

I understand the concepts behind code in the "old" call-manager.cpp and in the "new" dialer-manager.cpp are completely different, then these commits importing old code into new one are likely decreasing plasma-dialer's code quality... if developers have time, feel free to reimplement this in a cleaner way.

EDIT: needless to say, feel free to use this MR as a concept to be improved ;)