- Created a new detail namespace
- Moved shared module implementation details into the detail namespace
- Split dependencies so source files only rely on details in the detail namespace
- Removed all code from the juce_gui_basics.cpp file

This new factoring is a bit more typesafe, as it avoids casting
internals to void*.

It also allows cursors to scale appropriately on high resolution
displays on Windows.

As specified in https://specifications.freedesktop.org/clipboards-spec/clipboards-latest.txt, CLIPBOARD should be used for explicit cut/copy/paste operations over PRIMARY