Distro agnostic ffmpeg version test

OpenBoard.css: Add background color to QMenu, QPushButton and QComboBox

Fix wrong German translation: "Kompass" -> "Zirkel"

This reverts commit 63e5e8ec.

Fixes ui with dark system themes

reuse last used color for newer text boxes (only if applied globally) + recolor text boxes on background changes (white text becomes black and black text becomes white (like for polygons))

fixed an issue where font family styles of a block with the same font family were lost when changing size

improved scrolling behavior (ensure visible) + fixed issues on drag and drop due to constantly regenerated thumbnails...

In German, "Kompass" is the navigational instrument. The drawing tool is called "Zirkel."

Fix massive memory leak in UBFFmpegVideoEncoderWorker.

This reverts commit 94140cbc.

fix massive memory leak in UBFFmpegVideoEncoderWorker::writeLatest{Video/Audio}Frame()
resoulting from freeing av_frame without freeing its data.
fix #222

fix #217

This reverts commit 62a5ad33.

This reverts commit d5cf825c.