Fix some bugs (mem leak, layout, preview not showing colors)

1) memory leak when opening SchemeEditorDialog (is never deleted)
2) The color preview widget can be reduced in height to nearly zero,
   which makes no sense
3) The scheme editor does not reflect any color change
   in the preview widget
4) triple-redundant widgets in the scmeditorcolors.ui file
5) user unfriendly current tab when opening the SchemeEditorDialog
   (I want to change a color, so the color tab shall be active)
6) Unneeded and incorrect tab index enum (enum values were not
   corresponding to the index the tab was inserted)
7) The SchemeEditorDialog did not just only expand the color table when
   enlarging the dialogs height
BUG: 372309
BUG: 372407
REVIEW: 129902