fdo#74702 Moved EnableRTL() logic to specific classes where appropriate

OutputDevice::EnableRTL() is a bit of a mess. It uses a runtime
variable to see if it is using a VirtualDevice, and it uses a
dynamic_cast to see if the object is a Window or a Control!

I have made it virtual and moved the knowledge of class specific
functionality from OutputDevice to VirtualDevice, Window and Control
as needed. OutputDevice::EnableRTL() functionality is then called.

Also: small formatting change to outdev.hxx, also included a note
that WindowImpl is a pimpl in window.hxx.

Change-Id: I44b66601c4457fb2e0bbc1014fb7acf8f6942f80
Reviewed-on: https://gerrit.libreoffice.org/8721
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>