IDE: improve handling of newlines when colorizing

fixes #1216, #1219. This is a provisional solution, a more extended refactor (described in http://new-supercollider-mailing-lists-forums-use-these.2681727.n2.nabble.com/Post-window-colorization-pull-request-td7610633.html#a7611643) will change the behavior of cases like #1217

Conflicts:
	editors/sc-ide/widgets/post_window.cpp
	editors/sc-ide/widgets/post_window.hpp