Figure out a working hover popup for highlighters

[Feuermagier]

Hovering over a highlighter currently shows nothing. Instead, one has to hover or click on the gutter icon. The former has a multiple-second delay until the popup shows, which seems to be hardcoded in IntelliJ, and the latter is not very convenient.

There already is some working but commented-out code for a proper hover popup in HighlighterManager. The issue with it is that it conflicts with IJ's native popups (e.g. for JDoc and method info), which are always displayed on top of our popups.