Close Issue #172: Group titles does not change size when changing font size

Removes the hard-coded font size for the group heading style in the task list. This allows the group heading font size to match the user-selected font size.

Closes Issue #172 (Group titles does not change size when changing font size).

I only changed line 70 in MainWindow.xaml. I am not sure why the other files are part of this pull request, as I tried to pull in the latest changes from the main repo into my copy prior to developing the patch. If you have any guidance related to making these pull requests cleaner, please let me know.