Feature request: Add configure option to use Mod4 (or whatever) instead of Mod1 for window moving and resizing

[af7567]

After a lot of Googling for how to use the Windows/Super key instead of Alt for moving windows around, I finally decided to check the source code and saw that Mod1 is hardcoded (at least I think it is):

blackboxwm/src/Window.cc

Lines 1862 to 1872 in 7d9bb9d

	if (hasWindowFunction(WindowFunctionMove))
	blackbox->grabButton(Button1, Mod1Mask, frame.window, True,
	ButtonReleaseMask | ButtonMotionMask, GrabModeAsync,
	GrabModeAsync, frame.window,
	blackbox->resource().cursors().move,
	blackbox->resource().allowScrollLock());
	if (hasWindowFunction(WindowFunctionResize))
	blackbox->grabButton(Button3, Mod1Mask, frame.window, True,
	ButtonReleaseMask | ButtonMotionMask, GrabModeAsync,
	GrabModeAsync, frame.window,
	None, blackbox->resource().allowScrollLock());

It would be nice to be able to change that from some rc value like I have been able to in other window managers. I am also used to Win+middle button for resizing, so I guess being able to change the mouse button used in the above code using an rc file value would be good too :)