Make margins consistent on all sides for all KCMs

After frameworks/kdeclarative!25 (merged) is merged, there are still two remaining sources of extra margins in System Settings KCMs:

  1. Hidden horizontal spacing added by the internal KPageWidget
  2. Extra spacing between the KCM and the buttons below

Both are fixed now.

This causes one visual regression in the Shortcuts KCM which is fixed with plasma-desktop!174 (merged)

A few other KCMs which already have excessively large margins (Audio, KDE Connect, Date & Time, Spell Check) still look fine and are not urgent to fix.

For everything else, the result is a big improvement. All other KCMs, both QWidgets and QML alike, now have perfectly consistent margins. Some examples:





cc @teams/vdg @davidedmundson @iasensio

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