small UI change

when clicking the move up/down arrow on the craft soldier screen,
the mouse cursor will move up/down one line.
(applies to left click only)

not sure if this is the correct way to call SDL_WarpMouse(), or even if i'm
allowed to, but it works as far as i've tested*.


*this does not mean it works.
handling for edges.
Merge pull request #453 from Warboy1982/uifix

small UI change