Klipper: Introduce a "Popup" config page with options for the popup menu

With the check box options previously on the "Actions" page, plus the "Advanced" button (renamed to "Exclusions" to better reflect its function), a shadow of the enable option (as on the system tray popup menu) and the popup time (originally on the "General" page). This brings all of the popup menu configuration options together, and allows more space for the list on the "Actions" page.

Add some explanation text to the "Exclusions" list dialogue to explain its function.

This follows on from !1065 (merged) which introduced a similar style for the "General" config page.