This patch also makes Optional public to end users.

This change fixes an issue where UAD VST2 plugins would incorrectly draw
a menu at the top, rather than at the bottom, of the editor view.

This change fixes an issue where UAD VST2 plugins would incorrectly draw
a menu at the top, rather than at the bottom, of the editor view.

This includes Carbon support

Previously, the following plugins were causing issues when hosting their
editors:

- Softube plugins. I used Saturation Knob for testing, which crashed when
  deleting the temporary parent view.
- KORG Gadget series, which displayed a black screen after the temporary parent
  view was deleted.
- FabFilter Pro-C, which displayed at the wrong scale when opened on a
  retina display.

This commit partially reverts the changes introduced in 455e08da as having an intermediate HWND and reparenting the plug-in window was causing issues when hosting certain plug-ins such as u-he's Diva and Arturia's Mini V3

This patch also moves PluginHostType into juce_audio_processors in order
to allow hiding of function implementations.