libappfw: Added a line edit widget for Variable editing

Edits the value of a Variable as a text string.
Client|Master Server: Master cvars replaced with a Config variable

The three cvars for master server address, path, and port were
replaced with Config.masterServer.apiUrl that contains all the
information as a single string (standard web URL).

Fixes a problem with the master cvar values being cleared when
changing games.
UI|Client: Order of game subsets in the Games dialog

The Saved Games subset should appear below available games, because
this view is shown when the user selects "Switch Game", so that
should be the first subset listed.