Added a system to load news from MultiMC.org's RSS

Currently it doesn't show it anywhere in the UI. That's next.
Merge branch 'develop' of github.com:MultiMC/MultiMC5 into feature_news

Conflicts:
	CMakeLists.txt
Add news checking system and news toolbar
Merge branch 'develop' of github.com:MultiMC/MultiMC5 into feature_news

Conflicts:
	CMakeLists.txt
	gui/MainWindow.h
Implement the news button

Make the news button open the latest news post in the user's web
browser.

Also remove the webkit dependency. I was planning on using it to
display the news in a custom window, but we'll just use the user's
browser for now.
Merge branch 'develop' of github.com:MultiMC/MultiMC5 into feature_news
Tweak "More..." to "More news..."
Clean out old assets stuff, fix button hitbox regression