WIP: Rework JoinGroupchatWindow
This is a rework of the JoinGroupchatWindow.
What has been done:
- simplified code for server combobox and recent group chats popover
- adapted layout
- implemented many errors as popovers
- removed 'Auto Join' switch (autojoin is always active)
What could be done:
- find a way to replace the remaining ErrorDialogs (where server or room could be the error)