- added unit tests
- best performance timers used for each platform
- fixed an issue in which timer callbacks could drift

This is a breaking change - see BREAKING-CHANGES.txt

The current minimum-supported Xcode (10.1) includes the macOS 10.14.1
SDK, so APIs from macOS 10.14 will always be available.

The current minimum-supported Xcode (10.1) includes the macOS 10.14.1
SDK, so APIs from macOS 10.12 will always be available.

The current minimum-supported Xcode (10.1) includes the iOS 12.1 SDK, so
APIs from iOS 12 will always be available.

With this patch applied, the DemoRunner should build under MinGW, and be
(nearly) feature-complete compared to the MSVC build.

Specifically, when building with MinGW:
- Adds support for accessibility
- Fixes build issues in the juce_video module
- Fixes a link issue in the VST3 wrapper when VST3_CAN_REPLACE_VST2 is
  defined
- Adds support for the new-style native FileChooser
- Tidies up some other low-severity warnings

Known issues:
- Direct2D rendering is still not supported when building with MinGW due
  to ABI compatibilities.

This ensures the function IUnknown_GetWindow is declared in shlwapi.h
when building against the Windows 7 SDK.

The recent IFileChooser update to the JUCE FileChooser meant that
Windows build machines must be running Vista or newer.

For compilers other than MinGW, we were already requiring that the build
machine must be running Windows 8 or newer.