Use the rich text char format to store mentions rather than a separate structure in ChatBarCache.

This removes mentions from ChatBarCache and instead sets mentions as an anchor using the QTextCursor. Saving and restoring the chatbar text content is then done using QTextDocumentFragments which retain the rich text formatting.

src/libneochat/CMakeLists.txt

@@ -9,6 +9,7 @@ target_sources(LibNeoChat PRIVATE
     neochatroommember.cpp
     accountmanager.cpp
     chatbarcache.cpp
+    blockcache.cpp
     chatbarsyntaxhighlighter.cpp
     chatkeyhelper.cpp
     chatmarkdownhelper.cpp