We only have implementations of this on X11 and Win32,
so make it available as backend api there.
Update all callers to use either the backend api, or
just monitor 0.
We use a compilation symbol in our build to allow the inclusion of
specific headers while building GTK, to avoid the need to include only
the global header.
Each namespace has its own compilation symbol because we used to have
different libraries, and strict symbol visibility between libraries;
now that we have a single library, and we can use private symbols across
namespaces while building GTK, we should have a single compilation
symbol, and simplify the build rules.
The migration from GdkScreen's monitor API to GdkMonitor left out a way
to get the connector's name of a monitor.
While there's no real guarantee coming from the underlying graphics
system that the connector's name is stable, some system components may
use it to uniquely identify a monitor until the next plug in/out event.
Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
