Mnemonic: Emit sequence change also when it gets disabled
When merely changing enabled to false, the seuqence is reset but the property is not notified. This results in the Shortcut item on the control to still have the shortcut and might cause ambiguous shortcuts.
For example, in case of Kirigami ActionToolbar where there's usually two instances (icon and full) that are shown/hidden as needed, both buttons will remain active as disabling the mnemonic didn't work.
kf5, too?
This fixes not being able to use Alt+Letter for e.g. buttons in KCM headers.