Fixes#6411
- fix the order of buttons and the Skip button transparent background
- don't let the subHeaderItem eat vertical space when it's not visible
- layout/margins/spacing/linespacing fixes
Fixes#6387
TLDR; since this is a GridView, it needs a width/height set and then
centered on top of the parent component (as opposed to `anchors.fill`)
Also added a scroll indicator in case the window height is not tall
enough and fixed some small warnings
MishkaRogachev
Lukáš Tinkl
Patryk Osmaczko
Alexandra Betouni