[desktoppackage] Fix and enhance SliderHandles on a panel's Ruler

Apart from the interleaved clean-ups, this patch set fixes some issues and introduces usability features:

[desktoppackage] Add tooltips to SliderHandles on a panel's Ruler

This should make life easier for users who want to know what a control is doing before clicking it.

[desktoppackage] Reverse ruler handles on the left

Given current situation, I believe this is the best we could do. SVG has only two handle elements per orientation, so we should either mirror half of them using QtQuick transformations, or swap min/max. Both methods have their own disadvantages. This method (swapping) is used for preview in Plasma Theme Explorer, so it should be a "reference implementation".

[desktoppackage] Add double-click-to-reset behavior to ruler handles

Toggles between panel's current length and the total available size (usually screen).

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That's… probably too much for a single MR. But I'll give it a try.