backend/x11: improve output number handling

Created by: RedSoxFan

Same as #1615, but for the x11 backend

This improves the way the output numbers are handled for the x11 backend. Instead of using the number of active outputs plus one, the last used number is stored and new outputs will increment it. This fixes the situation where you start with one output, create a second, close the first, and create a third. Without this, both outputs will be X11-2, which causes an issue since the identifier will also be identical. With this, the last output is X11-3 and the outputs can be distinguished.