Auto highlighting of editor selections

This patch automatically highlights all occurrences of selections made in an editor.
This makes it easier to find for example all occurrences of a variable or function without having
to open the search bar. Leading or trailing white-spaces in the selection are ignored and
only selections that have no line-breaks are regarded.

The implementation is a bit hacky. A proper implementation would need to separate the
search functionality (and highlighting) from the corresponding GUI component.