kcms/kcm_connections: port to ItemDelegate & improve keyboard and screen reader a11y
The connection list is not keyboard and screen-reader accessible. This is difficult to fix at present, as it uses a custom delegate that does not follow our usual ways of implementing this, and also causes various visual inconsistencies such as a different hover feedback from other kcms.
The custom delegate doesn't seem to do anything we could not do with ItemDelegate and IconTitleSubtitle, so we port to that. There should not be functionality or major visual changes, except for some details becoming more consistent with ListViews elsewhere.
The main connection list has some keyboard navigation shortcomings, including the absence of visual feedback that makes keyboard-based use very opaque, and for some interface elements (such as icon-only buttons) the correct Accessible names and/or roles are not set.
This change makes keyboard navigation of the list (and whole sidebar) work mostly as expected, and exposes the the correct roles and/or existing strings to the Accessibility framework where necessary.
No string changes (so far); we could consider this for 6.4 as it massively improves accessibility of this kcm.