Add a "hide-docks" config and connect the Windows->Hide docks menu
item to it. Also connect the image window to the config property so it
can hide/show its docks when it needs to.

Also add and use a utility function
gimp_image_window_keep_canvas_pos() to ensure that the image in the
window remains fixed when toggling visiblity of docks. One problem:
When GimpDrawTool is active on the canvas, there is flicker. The end
position is correct though.

Also add regression testing for this fix to test-ui.c