Highlight Parent Directories in Treemap upon Middle Click

[shundhammer]

This was inspired by issue #176.

In the treemap, a middle click now also behaves like a left click (including Ctrl+click); but it also displays white frames indicating the parent directories of that item, and everything that does not belong to that directory branch is covered by a transparent overlay.

Left-click anywhere in the dimmed area, or click the same tile again, to get rid of the rectangles and of the transparent overlay.

Hover over any of the white frames to get a tooltip with its exact path.

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The previous middle click action was "Up one level"; this is still available, of course, but no longer on a middle mouse click: It's in the tool bar, in the "Go" main menu, in the context menu and with the Alt+Cursor up key combination.