Allow taking area screenshots after timeout.

This removes the restriction that area selection screenshots could not be used in combination with timeouts. That made it impossible to screenshot e.g. tooltips and context menus as area screenshots.

Fixes #43 (closed) and allows removal of the workaround to disable the timeout option.

Side note: I originally tried to store a pointer to GdkRectangle in _ScreenshotApplicationPriv, but for some reason not entirely clear to me this seemed to get freed after g_timeout_add or g_idle_add. Hence I changed this to statically store the GdkRectangle, which I think is ok for holding four integers.