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/neochatroom.h

@@ -47,8 +47,15 @@ public:
     /// Convenient way to get the last event but in a string format.
     ///
     /// \see lastEvent
+    /// \see lastEventIsSpoiler
     [[nodiscard]] QString lastEventToString() const;
 
+    /// Convenient way to check if the last event looks like it has spoilers.
+    ///
+    /// \see lastEvent
+    /// \see lastEventToString
+    [[nodiscard]] bool lastEventIsSpoiler() const;
+
     /// Convenient way to get the QDateTime of the last event.
     ///
     /// \see lastEvent