Changed "Downloads" and "Addons - Legacy" sections to not search mountable games, etc.

Downloads from mountable games are not mounted by Garry's Mod, meaning that models and stuff from those locations cannot be used in-game. So we just do not show such content.

Same goes for addons.

This will also improve filesystem performance depending on how many mountable games the player has enabled.

Maybe slightly faster start up times, who knows.

* Reload the Browse => Addons/Games for Materials and Sounds when the game content remounts (Will only work on beta branches and in the next game update)

* Relaxed restrictions on Material section, allowing you to try to use materials with the material tool regardless of whether the addon thinks it will work or not
* Right click menu icons for Materiials and Sounds
* Moved addon info from an obscure spot into the Utilities menu and changed it up a bit

* Fixed a potential Lua error when someone does something stupid like calling GM:PopulateContent with incorrect/missing arguments

* Added "Downloads" tree to Models, Materials and Sound browsers, shows content downloaded from servers via FastDL
* Updated the names to be translated to other languages where possible.
* Added OSX/Linux notice about lowercase names to Info thingy.

Fixed a problem with Material and Sound browsers introduced by last update.

* Fixed a bug with materials not saving in spawnlists
* Organized all addons/games alphabetically
* Prevented workshop addons from showing up in sounds/materials browser if they don't have sounds/materials in them

* Prettify game names in Browse Materials and Browse Sounds
* Removed debug output