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.cpp

@@ -147,6 +147,19 @@ const RoomMessageEvent *NeoChatRoom::lastEvent(bool ignoreStateEvent) const
     return nullptr;
 }
 
+bool NeoChatRoom::lastEventIsSpoiler() const
+{
+    if (auto event = lastEvent()) {
+        if (auto e = eventCast<const RoomMessageEvent>(event)) {
+            if (e->hasTextContent() && e->content() && e->mimeType().name() == "text/html") {
+                auto htmlBody = static_cast<const Quotient::EventContent::TextContent *>(e->content())->body;
+                return htmlBody.contains("data-mx-spoiler");
+            }
+        }
+    }
+    return false;
+}
+
 QString NeoChatRoom::lastEventToString() const
 {
     if (auto event = lastEvent()) {