Update settings dialog to check RegExp syntax immediately

[Nathan Phillip Brink]

In #3, I added a settings dialog to allow specifying regular expressions to match. I attempted to add some RegExp error handling. It is asynchronous and only triggers when messages match.

Instead, if there is a RegExp parsing error, it should be validated in the settings dialog and prevent saving the settings. The asynchronous notification logic can be removed. This should simplify things greatly.

As part of this, the parsed expressions can be loaded immediately and cached instead of reparsed each time messages are checked.