Steam Client accommodates Beta Update: Opportunities around fixes and adding.

Henceforth since February 7, fans of Steam Client can enjoy its Beta Update.

In fact, for those unfamiliar, Steam is the entertainment platform where you can via a client: play, share or propose the video games.
Therefore we have :

- Added third party Mod support for games on new content system
- Fix screenshots sometimes not showing up in screenshot manager when copied from another Steam installation
On Mac we noted:
- Add partial support for Retina displays
- Fix bug w/tooltips showing through Windows
Finally on Linux, developers have featured:
- Fix executable file system check bug when installing games
- Fix runtime crashes in libappindicator, XCreateIC
- Fix launching Steam with STEAM_RUNTIME=0
- Fix problem connecting to X11 display when XAUTHLOCALHOSTNAME is set
- Reduced the size of the Steam Runtime by 50%
- Added license to allow repackaging for other Linux distributions