Handle wxEVT_SYS_COLOUR_CHANGED in wxAUI classes

Update the colours used when the system theme changes.

This is especially important to use the colour scheme compatible with
macOS 10.14+ dark mode.

Note that this commit is best viewed with "git diff --color-moved".

Closes https://github.com/wxWidgets/wxWidgets/pull/916

docs/changes.txt

@@ -141,6 +141,7 @@ All (GUI):
 - Add wxJoystickEvent::GetButtonOrdinal() (Mick Phillips).
 - Add wxGraphicsContext::GetWindow() and implement wxGraphicsContext::GetDPI().
 - Add wxToolbook::EnablePage() (Stefan Ziegler).
+- Adapt AUI colours to system colour changes (Daniel Kulp).
 
 wxGTK: