S3 bucket listing support and network code refactors.

* Adds support for listing all objects in an S3 bucket.
* Renames a bunch of network related classes (Download->Action)
* Net actions now have static constructors
Merge branch 'develop'
Fix problem with instance list not using the instance folder path
Valgrind-checked early application start
BLIND CODE, BLIND CODE HERE! WHERE IS THE COMPILER!?!?!

It choked on missing headers, maybe?
Add selectable message box helper, use it, make login GUI error label selectable
Don't make LabeledToolButton selectable
Move "Forget" button to a sane place
Make ConsoleWindow have Qt::Window flag (adds minimise/maximise buttons to titlebar)
Customizable folder browsing is same as for the instance folder. Syste Proxy usage.
Merge branch 'develop' of github.com:MultiMC/MultiMC5 into develop
Merge branch 'develop'
Hack for Intel graphics drivers on Windows.
List available proxies on start
Merge branch 'develop'
Revamp Linux build instructions for Qt Creator
Main window geometry and state saving/reloading
Merge branch 'develop'