Add ability to specify a "defaultPath" in a field config, which will set the default contents of the path textbox

[schultzcole]

This could be useful for cases where a particular field is very commonly used as the default.

For instance, in an Active Effect change key, the vast majority of the time, people will be using a key that begins with data.. There should be a way for a field config to specify that it should begin with that path already entered when the Autocompleter is created.