Support displaying spoilers

This adds support for displaying recieved spoilers, but not sending
them. Spoilers are displayed as a black rectangle, and can be clicked on
to be revealed. If the last message in a channel was a spoiler, it is
not shown on the left sidebar.

The spoiler blackening is done in CSS, but to check if a message
contains a spoiler for determining if it should cause a different cursor
to be displayed and if it should be shown in the sidebar, a simple check
of if the message contains "data-mx-spoiler" is used.

src/roomlistmodel.cpp

@@ -364,6 +364,9 @@ QVariant RoomListModel::data(const QModelIndex &index, int role) const
         return room->highlightCount();
     }
     if (role == LastEventRole) {
+        if (room->lastEventIsSpoiler()) {
+            return QString();
+        }
         return room->lastEventToString();
     }
     if (role == LastActiveTimeRole) {