ElevationProfile: Refactoring

Refactoring of the ElevationProfile plugin code.
* axis handling moved to a dedicated class,
no more double-code for X and Y scale calculation
* try to generate 'nicer' interval ticks, e.g. 42.5 instead of 42.444
* "double click -> jump to location" now works in zoom mode, too
* minor optical changes
* shortened the "Elevation difference" text to not get cropped in
Marble Touch
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