The change introduced in 92f350e617 led to
a regression in the MoveEndpointByUnit function. In this case, a
backward movement by a single character *should* move the endpoint as
long as it remains within the text range.

The issue addressed by the faulty commit is better fixed by
special-casing the 'character' unit case in the ExpandToEnclosingUnit
function.

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.