Move and rework DoubleInputDialog to DestroyMucDialog
DoubleInputDialog was used in just one case: Destroying a group chat. Needing two input fields in a dialog is a rare case, so I reworked this dialog to a dedicated
DestroyMucDialog. I left the arguments for labels, so one could distinguish between differnt types of group chats in the future (e.g. group chat/channel or participants/members).
- Both entries for
alternate venueare hidden by default, but can be expanded
- Pressing Alt+R mnemonic will expand the expander to show the entries
- Pressing Enter now automatically activates the Destroy button