Clean up code style of public QML components

Also for every RowLayout and ColumnLayout without an explicit spacing,
use Kirigami.Units.smallSpacing, which is only 1 pixel away from the
defaults of 5 px. We should always specify some spacing, because
defaults do not align with anything in KDE HIG.

Changes in this patch are mostly:

- get rid of import versions (leaving only Kirigami 2, as it used be
  problematic);
- change QtControls import alias to a more conventional QQC2, and get
  rid of QtLayouts alias;
- drop unnecessary semicolons after property/binding declarations;
- add lots of vertical whitespace and shuffle some properties around for
  readability;
- use const and let keywords instead of var in some code blocks.