Add some basic highlighting to build output textedit and use editor theme colors

This is a proposal to add coloring to the build output plaintextedit to make it a bit nicer. At this point there's not much coloring and it is only done for "make output" (which is what i use 99% of the time, I know ninja is better but just too much used to it :)).

I just wanted to share the concept with you guys to see if this is okay and whether we should proceed further with this. It was also a great oppurtunity to explore the new "configChanged()" and theme API.

Additionally, I propose that we don't throw away the plaintextedit output more usable, as in, clicking on a file takes us there and maybe something more?

@sars @cullmann @dhaumann