Ribbon bar drop down arrow disabled colour

[wxBen]

A while back an option was added to the ribbon bar to use a disabled text colour for disabled items. That works well, except buttons with drop down arrows still draw the arrow in the original non disabled colour, making them stand out.

This patch extends the use of the disabled colour to the down arrows as well.

This change only takes effect if the user sets a disabled colour, since it defaults to the regular colour.

[vadz]

Would it be possible to avoid code duplication by adding a function for drawing this arrow and reusing it?

[wxBen]

I dislike the code duplication as well. Is this version more acceptable?

[wxBen]

DownArrowColour.diff

[vadz]

It's better, thanks, but as long as we're doing it like this we could reduce the duplication even a bit more, will commit soon.

[VZ]

(In [73086]) Use disabled colour for the dropdown arrow of disabled ribbon.

Previously the normal colour was used for the arrow in MSW art provider even
if the ribbon was disabled.

Closes #14864.