Isolated algorithmic improvements.

* Fix inefficient STL use around Parser::UTF8Parser.
* Reduce typeid() usage, change some of it to a virtual method
* Do multiple-line scrolls as a single move

src/terminal/terminalframebuffer.h

@@ -326,8 +326,8 @@ namespace Terminal {
 
     void apply_renditions_to_current_cell( void );
 
-    void insert_line( int before_row );
-    void delete_line( int row );
+    void insert_line( int before_row, int count );
+    void delete_line( int row, int count );
 
     void insert_cell( int row, int col );
     void delete_cell( int row, int col );