Viewport isn't fully repainted when line widget is resized even after refresh

[njx]

1. Place https://gist.github.com/4498433 next to the lib folder containing codemirror.js and open it
2. Click the "Resize Widget" button

Result: The lines below line 11 aren't repainted until you scroll or do some other action. This occurs even though we explicitly call refresh() after resizing the widget. Seems like either refresh() should force widget heights to be rechecked, or we should have some way for a line widget to notify CM that its height has changed.

[njx]

Similar to #1102.

[marijnh]

Thanks for the helpful test case. I've pushed first a fix that makes refresh behave correctly in this situation, and then another one that adds a changed() method to line handlers, which provides a finer-grained (and cheaper) way to notify the editor when a widget changed.