Sourcemod support

[PSebs]

There's some sourcemods that are not on steam like Open Fortress. They do not appear in the list of non-steam games to choose from as of now.

[Jozen Blue Martinez]

Leaving some notes here for future reference or if someone wants to submit a PR:

1. Find path: HKCU\SOFTWARE\Valve\Steam\SourceModInstallPath
2. To get names read gameinfo.txt file in each mod. Name is under GameInfo->game.
3. Sort mod dirs alphabetically and index them by number.
   For example if we have 2 dirs "crates" and "mapbase", "crates" it would be 1, "mapbase" would be 2, and so on.
4. Generate new appid: newAppId(index).
5. Save the usual way: appid.png, appid_logo.png, appid_hero.png etc.

For icons, probably shouldn't mess with game files but just in case:
Icon needs to be a tga saved in the mod dir then edit gameinfo.txt: GameInfo->icon value with relative path to icon.

[Manos Paterakis]

Added support for source and goldsource mods. ef37e34