- 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

Previously, setting a new look and feel would create a new missing-items
button, but would not add it to the toolbar or set the onClick callback.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.