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.