Port config dialogs from .ui to QFormLayout

* C++ instead of .ui files.
* Put every control in a QFormLayout row.
* Where possible, use only a single layout, i. e. no nesting.
* Use rows with an empty QLabel as horizontal spacer.
  Content grouping by whitespace instead of group boxes.
* Where group boxes were used for labeling tool managers,
  use a <h3> QLabel instead.
* Where group boxes were used for labeling control groups,
  drop the label or label only the first control (like in Dolphin).
* Use combo boxes instead of radio button groups.
  (I don’t know how to connect radio buttons to KConfigWidgets...)
* Don’t use checkboxes as label, that doesn’t lay out correctly.
  Instead, put the checkbox in an own row, although it controls the same feature.

Some of the changes are not perfect yet, so I opened
graphics/okular#45 to outline further improvements.