[sddm-theme] Start moving from QQC1 to QQC2

Summary: Components that used QQC1 in `sddm-theme/` (excluding the symlinked `components` folder) have been changed to use QQC2.

Test Plan:
Ensure no loss of functionality, style, or bugs from the port of QQC1 to QQC2.

{F7256235}

Reviewers: #plasma, #vdg, davidedmundson, filipf

Reviewed By: #plasma, #vdg, davidedmundson, filipf

Subscribers: ndavis, GB_2, ngraham, filipf, davidedmundson, plasma-devel

Tags: #plasma

Maniphest Tasks: T10958

Differential Revision: https://phabricator.kde.org/D21815