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
Maybe break updater even more?
Merge https://github.com/robotbrain/MultiMC5 into develop
Fix various LiteLoader related bugs.
Launch an editor for the custom.json file. Also allow overriding of the system default editor
Use /usr/bin as default on linux
Don't use stuff that's new in 5.2 if we don't compile with that
Notify the user of any errors
Fix spelling mistake
Merge pull request #28 from 02JanDal/feature_editor

Button for opening an editor for the custom.json file
Make the java browse button look like the other browse buttons
Clarify code style for contributions
Merge pull request #29 from 02JanDal/java_browse_button

Make the java browse button look like the other browse buttons
Refactor icon lists heavily

* Icon list now uses a filesystem watcher for updates
* Icon folder is user-customizable
* All the little details. ALL OF THEM.
Fix silly bug with icon not changing on the toolbar when the icon key changes.
Enable high DPI support on OSX
Clarify terms of redistribution and our wishes under the Apache license
Add the redistribution/fork text to the About dialog.
Tweak credits in about dialog