Invert menu icons when white icon on light background or black icon on dark background

See https://github.com/LMMS/lmms/issues/3349#issuecomment-279477069:

> I just read a bug report with users complaining about this identical behavior, except when in dark mode. Apparently there's an unmerged feature to help... https://codereview.qt-project.org/#/c/115120/
>
> Linked from https://bugreports.qt.io/browse/QTBUG-42109.
>
> For the short-term, we can use a quick hack, such as inverting the color palette for these icons. http://doc.qt.io/qt-5/qimage.html#invertPixels

When #3349 is closed this issue will apply to new default theme icons for templates and project files only on macOS/OS X.

Start with issue #3349 and its linked comment, then read Qt's QImage::invertPixels documentation and the linked Qt bug reports. Confirm which default template and project-file icons are affected on macOS; done means white icons are not shown on light backgrounds and black icons are not shown on dark backgrounds.

Written by the indexing model from the issue text.