Burndown Chart

- Now renders (and wraps if necessary) a chart title, with additional
  filter description.
- Removed more redundant code.
- Fixed divide-by-zero bug when the filter resulted in zero matches.
- Made the burndown commands read-only, so that filters apply.

src/Cmd.cpp

@@ -247,6 +247,9 @@ bool Cmd::isReadOnlyCommand ()
       command == "history.annual"                                            ||
       command == "ghistory.monthly"                                          ||
       command == "ghistory.annual"                                           ||
+      command == "burndown.daily"                                            ||
+      command == "burndown.weekly"                                           ||
+      command == "burndown.monthly"                                          ||
       command == context.stringtable.get (CMD_CALENDAR,    "calendar")       ||
       command == context.stringtable.get (CMD_COLORS,      "colors")         ||
       command == context.stringtable.get (CMD_DIAGNOSTICS, "diagnostics")    ||