Improve the Jitsi meeting button UX

Widgets support is incredibly useful and so is this button, but it has a
few problems. The most obvious is that it's still enabled even if you
don't actually have the permission to start Jitsi meetings, so I fixed
that. I also made sure it's hidden when viewing spaces too.

Another problem is that you can't easily tell if a meeting is currently
in progress either, nor do we have a good icon for that in Breeze. So I
changed the tooltip and colored the icon in this case.

The final problem I fixed is something not exclusive to NeoChat, but
generally all chat applications with this feature - there's no
confirmation! To stop "butt-dialing" random people or rooms, I added a
prompt before starting or joining a meeting.

src/app/CMakeLists.txt

@@ -103,6 +103,7 @@ ecm_add_qml_module(neochat URI org.kde.neochat GENERATE_PLUGIN_SOURCE
         qml/ReasonDialog.qml
         qml/NewPollDialog.qml
         qml/UserMenu.qml
+        qml/MeetingDialog.qml
     DEPENDENCIES
         QtCore
         QtQuick