Simplify the code by not making this function pure virtual as all the
ports except MSW had to override it just to return an empty string.
Instead, just return empty string by default as it's not critical to
force the derived classes to override this function.
Instead of forwarding to these functions from wxDisplay implementation
in wxUSE_DISPLAY==0 case, make the functions themselves wrappers around
wxDisplay, which may, or not, depending on the platform, have a simpler
implementation in wxUSE_DISPLAY==0 case, but is always available in any
case.
As part of this change, only use src/osx/core/display.cpp in macOS
builds, not iOS ones and update the Xcode project accordingly too.
This cuts down on code duplication, especially in wxGTK, and facilitates
further additions to wxDisplay API.
Avoid a heap allocation on every wxDisplay creation by caching the
wxDisplayImpl objects once they're created.
Currently we never invalidate the cache, but we should add a way to do
it in the future.
This speeds up wxDisplay::GetGeometry() benchmark by a factor of 4.
