[Qt-desktop] Add support to USB gamePad

New dialog window to select USB gamePad to play and configure/test the mapping.

Qt don't have code for handling gamepad, so it use SDL for multiplatform. Qt check if SDL lib is installed at compile time to enable the code. So those who don't want SDL and gamepad can always build. If SDL not present, a popup message is displayed ingame saying the gamepad is not avaiable if the user try to access the config dialog.