FormLayout

Summary:
Add a new component called FormLayout, which aims to
behave similar to Qwidget's QFormLayout: 2 columns
with labels aligned accordingly to the HIG
when the available space is too small, collapse
to one column only, intended especially for mobile.

Buddy items don't yet have keyboard shortcuts, but apart text decoration, they shouldbe easy to implement.

intended usage is based on the FormData attached property:
FormLayout {

 TextField {
     FormData.label: "Label:"
 }
 //separed by a line, no titles
 Kirigami.Separator {
     Kirigami.FormData.isSection: true
 }
 TextField {
     FormData.label: "Label2:"
 }
 //separed by a line, and a title
 Kirigami.Separator {
     Kirigami.FormData.isSection: true
     Kirigami.FormData.label: "Section title"
 }
}

Test Plan: the gallery has a FormLayout page used to test it

Reviewers: Plasma, Kirigami, hein, davidedmundson

Reviewed By: Plasma, hein, davidedmundson

Subscribers: broulik, colomar, ngraham, davidedmundson, plasma-devel

Tags: Plasma, Kirigami

Differential Revision: https://phabricator.kde.org/D8641