ui/cocoa: Perform UI operations only on the main thread

The OSX Mojave release is more picky about enforcing the Cocoa API
restriction that only the main thread may perform UI calls. To
accommodate this we need to restructure the Cocoa code:
 * the special OSX main() creates a second thread and uses
   that to call the vl.c qemu_main(); the original main
   thread goes into the OSX event loop
 * the refresh, switch and update callbacks asynchronously
   tell the main thread to do the necessary work
 * the refresh callback no longer does the "get events from the
   UI event queue and handle them" loop, since we now use
   the stock OSX event loop

All these things have to be changed in one commit, to avoid
breaking bisection.

Note that since we use dispatch_get_main_queue(), this bumps
our minimum version requirement to OSX 10.10 Yosemite (released
in 2014, unsupported by Apple since 2017).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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