Reset dialogs, improve error handling, minor changes

Covers:
* #219 - Reset dialogs when opened. Turns out this was a bug in `StringProperty.prototype.reset` from when we added originalValue.
* #220 - Remove error message when no action/event is selected for edit
* #221 - Open Project Tree on New/Load project

While working on #219 I also improved the error handling. There were bugs that occurred when dropdowns didn't have an option. Also, there was no way to report errors for those dropdowns, so I added those. Finally, for PropertyAction, you can no longer save the action if the properties have errors.