Allow dragging of modal gumps

[wench]

Exult replicates original behaviour and does not support dragging modal gumps to move them. Geneally this is fine but it can cause trouble if the size of the game area is less than 320x200. This was never a concern for the original as it always ran at 320x200 but exult can run at other resolutions. In particular the apply button on the video options gump is right at the bottom so it can go off screen if the game area is less than 200 pixels high so if for some reason the game area is too small it may be impossible to change settings to something better from within exult.
I got thinking about this so people could run exult at 1080p (actually 1600x1080) and have perfect aspect correct scaling of a 320x180 game area with 6x vertical and 5x horizontal scaling factors. Losing those 20 vertical pixels shouldn't cause problems in game (it ends up being 10 pixels at top and bottom so a fraction over a tile) but might for modal gumps. For 320x200 perfect aspect correct scaling a 1200p display is needed and while I have 2 of them (16:10 workstation displays) they are not particularly common so I want to make this a useful option for all.

If anything adding UI consistency allowing all gump types to be draggable would be nice
This would also allow us to have oversized gumps bigger than 320x200
I would add a config option to disable this behaviour for purists.
Even if disabled by config I would probably always allow it for gumps that clip offscreen
An alternate idea would be to add edge scrolling for offscreen gumps, but as this is not a standard UI interaction in u7 it seems like a bad idea. (crazy idea add edge scrolling to the game world too)
I am currently doing a lot of things to gumps while working on #502 so I will probably include this in the commits for that.