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