Remove checkable, checked and enabled properties from FormLayout/FormData
This removes all three properties from FormData attached type and their usages from FormLayout component. The reason is that they are not used, require considerable amount of code to keep it from falling apart, and requires FormLayout override/style CheckBox component which is really not its job and which we miserably failed to maintain consistent at least with qqc2-desktop-style (and which also is unreadable in RTL mode).
The checkable/checked properties are not used anywhere throughout Plasma software. The Kirigami.FormData.enabled only appears twice in plasma-firewall for CheckBox labels (where it doesn't do anything, because according to my observations disabled labels don't change colors in qqc2-desktop-style).
For those who wonder what this is all about, here's how it looks like with various combination of FormData properties set:
(Using this snippet: $2623)
That's quite weird layout and alignment on its own. And with RTL mode set it goes ba-da-boom!:
Draft state because Kirigami Gallery and plasma-firewall need to be ported away from it first.