Expand mouse control methods

Replace the [sdl] `autolock = true/false` configuration setting with [sdl]
`capture_mouse = ...` with a two-value setting.

The first value defines how the mouse is controlled:
- onclick:       The mouse will be captured with a click inside the window.
- onstart:       The mouse is captured immediately on start (similar to real DOS).
- seamless:      The mouse will move seamlessly in and out of DOSBox and cannot be captured.
- nomouse:       The mouse is disabled and hidden without any input sent to the game.

The second value defines how middle-clicks are handled:
- middlegame:    Middle-clicks are sent to the game (not used to uncapture the mouse).
- middlerelease: Middle-click will uncapture the mouse when windowed (not sent to the game).

Middle-clicks are sent to the game when fullscreen or when seamless control is set.

The default setting of "onclick middlegame" reproduces DOSBox's existing behavior.

include/video.h

@@ -69,8 +69,4 @@ bool GFX_SDLUsingWinDIB(void);
 void MAPPER_UpdateJoysticks(void);
 #endif
 
-/* Mouse related */
-void GFX_CaptureMouse(void);
-extern bool mouselocked; //true if mouse is confined to window
-
 #endif