macOS: basic event handling

[greatest-ape]

Adds barebones handling of mouse down events on macOS (#51) with a lot of code from https://github.com/antonok-edm/vst_window. The open_window example works.

Possible problems that I can think of:

• No reference to the created Window is stored in the WindowHandle, which means it can't be accessed later and cleanup must be handled by macOS (?).
• Parent window functionality has not been tested
• AsIfParented support is not implemented

If this looks like something we could make use of, I can have a look at implementing other events as well.

[greatest-ape]

Several problems have been identified and fixed. Some remaining work to be done is:

• [x] Get CursorMoved events when user is not dragging
• [x] Test if parented views work. https://github.com/greatest-ape/baseview_test_vst2 is useful
• [x] Test if AsIfParented views work (AUv2 only) Skip testing AsIfParented for now, there is no viable way of creating AU plugins at the moment
• [x] Does the NSApp need to be created before setting up the non-parented window and get passed into the WindowHandle for _app_runblocking? Probably, see rutabaga and cocoa docs
• [x] remove keyboard event handling, that should be done in other pull request; however, keep acceptsFirstResponder code
• [x] Setup tracking surface for drag to work outside of the window cursor entered and exited events.
• [x] Mouse coalesce stuff? Rutabaga link
• [x] preservesContentInLiveResize = NO do this like in rutabaga
• [x] ask for first click to register, rutabaga does this
• [x] clean up the class when window closes / view deallocates. Actually, check that dealloc function is called at all (nope). wrl suggests checking for backtraces in retain to see if they come from my code. Clean up should be done in release function, look in rutabaga. High refcounts in release (also retain?) mean that view doesn't get deallocated. This could all be due to lack of proper autoreleasepools. Check where they are used in rutabaga. ”yeah whenever you have an alloc you need to either
  1. call autorelease on it and have an autoreleasepool in scope
  2. or make sure you later call release on it”