Add display/window/screen_id configuration

[marcelofg55]

This new configuration option can be used to start a project on a specific screen.

In combination with #23799 it is now easy to start projects at the last screen used, saving the screen_id with something like:

	var cf = ConfigFile.new()
	cf.set_value("display", "window/screen_id", OS.get_current_screen())
	cf.save("user://override_ps.dat")

Note 1: This also adds a No management option to run/window_placement/rect so that the editor does no management with the window position. Otherwise the display/window/size/screen_id is overridden by the editor (if the project is run inside the editor).

Note 2: By default now Windows, OS X and Linux create the window centered. Previously only Windows and OS X centered the window when it was being created.

[mhilbrunner]

Conflicts need to be fixed :)

[akien-mga]

Moving to next milestone as it's not in a mergeable state, so it's too late to include in the imminent 3.2.

[aaronfranke]

I believe this PR is no longer relevant thanks to the OS and DisplayServer split. DisplayServer has methods such as window_set_current_screen which should allow what this PR is trying to accomplish. Therefore, I'm closing this.