Add M-x customize parameter to set the screen position for the controls toolbar

[genegoykhman]

I found the default position of the dap-ui controls toolbar interfered with the top line of my preferred layout, so this PR turns the position of the controls toolbar into a customizable variable dap-ui-controls-screen-position. The variable accepts, as a value, the positioning functions used by posframe.

For example:

(setq dap-ui-controls-screen-position #'posframe-poshandler-frame-top-left-corner)

will position the controls toolbar in the top-left corner of the screen (my preference).

The default value remains #'posframe-poshandler-frame-top-center so current users will not be affected.