Allow inlineEditor dirty field calculation to be overridden.
The current dirty field calculation in inlineEditor is fine when dealing

with textareas, which have no concept of dirty state to begin with, but

custom text fields may be smarter than that. In those cases, we want

callers to be able to compute a dirty state on their own.
Now, options.isFieldDirty can be provided to return whether or not an

editor is dirty. The default implementation compares text, as it's

always done.

Tested this with an accompanying Review Board change, and noticed that

the typing lag I've seen in the Markdown editors went away.