Selecting text is slow if selection is synced with clipboard

Some platform context: Linux has both a selection and a clipboard, and on e.g. KDE it is possible to sync them. That means that selected text ends up on the clipboard.

https://codereview.qt-project.org/c/qt-creator/qt-creator/+/637170 added some watching logic for the clipboard.  When selecting a block of text, that causes (for a large enough selection) hundreds of hits. Those hits all try to retrieve the text (only to check if it's) empty, which in turn causes some UI stutter (at some point we end up polling the clipboard, wich takes time).

Could we maybe try to debounce the checking?