Separate some back-end from Spectacle into a library

[Igor Mironchik]

Hello.

I'd like to make a request to separate some back-end from Spectacle into 
a library. At least ImagePlatformKWin.h functionality may be reused by 
other developers on KDE platform for some third-party screen capturing. 
In can of move KDE to Wayland only this can be useful, at least for me :)

This should not be a separate repository but in CMake should be splitted 
back-end in some library. Hm, and an option for building back-end only 
could be very great. :)

What do you think?

Thank you.