Automatic acquisition of Qt sources

[blizzard4591]

Dear sijk,

in my fork I implemented a download feature in CMake that automatically retrieves the correct headers/sources for the locally installed version of Qt5.
This makes building much more convenient (i.e. just do cmake && make).

I dont know whether my other changes are interesting for you, but some parts might be - please do tell me if you would like to merge some!

Cheers and thank you for this nice wrapper :)