Upgrade master with changes from v4 branch

* Custom folder and VPK loading (your own materials/models/etc)
* Loading default resources from VPKs (like skyboxes and sounds)
* Responsive UI while TF2 is running
* Reduced start-up times
* Automatic updates

Will merge as soon as the branch is stable. To be updated:
* Linux support
* Demo editor (renewed VDM interface)