Linux/opengl: Add Frame when running in Window Mode

Is your feature request related to a problem? Please describe. When running the game in windowed mode there is no frame, which means the game cannot be moved around the screen easily.

Describe the solution you'd like Window frame added when in Window mode. Mouse behaviour as below: When 'in game' the mouse should be 'captured' from the OS, but when you're on the Menu screen after pressing ESC, the mouse should be able to leave the frame as per operating system normal. Excuse my not knowing the terminology.