Align lock icon with bold message text; reduce overall size of dialog

Summary: Adjusted spacers in UI file to properly align icon with message text. Also attempt to minimize white space in dialog box by hiding UI elements until they are needed. (Set `height` to `1`, then restore to original layout size.)

Test Plan:
- Compile polkit-kde-agent-1 with patch
- Trigger a root password request window (launch Synaptic, try to change SDDM in System Settings)
- Ensure that lock icon is aligned with bold headline text

Reviewers: davidedmundson, ngraham, abetts, #frameworks

Reviewed By: davidedmundson, ngraham, abetts

Subscribers: stikonas, bruns, ltoscano, broulik, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12311