kcm: Appearance tweaks
Move "Configure Appearance" button into header actions
We don't have a "preferences-desktop-theme-symbolic" icon in Breeze
at the time of writing and the non-symbolic version out of place
in the header bar. Switching to "edit-image-symbolic" instead,
which works surprisingly well as picking wallpapers takes up most
of the Appearance sub-page.
Use full width for both duration comboboxes in the form layout
It's more consistent this way, and the extra width for the first combobox hurts neither looks nor usability.
Before: 
After: 