ScreenChooserDialog: overhaul interaction

Requires frameworks/kirigami!1432 (merged).

Right now this dialog has two modes: multi-select and single-select. The active mode is currently unclear, since nothing communicates whether the screen and window cards are multi-selectable or not. And in both modes, cards have no hover feedback, which is inconsistent with the KDE style for interactive UI elements.

This commit fixes those issues by refining the UI in various ways:

• Add hover feedback
• Use the new "I have a checkbox now" styling for checkable cards when in the multi-select use case
• In single-selection mode, select on single-click and hide the Share and Cancel buttons, since they're no longer needed. This matches the behavior of the App Chooser dialog, which works in the same way

BUG: 479925 FIXED-IN: 6.0

Here's how it looks now in the two modes:

Single-select mode	Multi-select mode

@teams/vdg @teams/usability please only review the actual changes in this MR and don't propose additional unrelated ones; I know there are still some rough edges, but those will be done separately.