[kcms/keys] Port sidebar list item to Kirigami.BasicListItem

This commit ports the Shortcuts KCM's sidebar list items to use
Kirigami.BasicListItem, which fixes a binding loops and some bugs
related to incorrect colors caused by the custom content item, and
gets rid of another bespoke list item implementation.  The embedded
hover ToolButtons are changed to regular Buttons to avoid losing their
hover appearance entirely due to differences in BasicListItem's
coloration semantics.

The result has minor visual differences but is functionally the same.

BUG: 436816
FIXED-IN: 5.23