Tweaks to resizing handle

[kboone]

Here are a couple tweaks to the resizing handle. These are mainly designed to make things behave reasonably when matplotlib rendering times are longer.

First, ignore mouse events when resizing. I found that when there are delays in image resizing, the cursor sometimes crosses the image which triggers a ton of mouse events to update the x/y text at the bottom of the image and everything grinds to a halt.

Second, when there is a long delay in rendering, with the current implementation it is very hard to see where you are dragging the figure to because the boundaries of the image only update whenever the image itself does. I changed this so that the boundaries update on the client as you drag the mouse. This is my preferred behavior, and is also what the notebook backend currently does.

[martinRenou]

That is indeed a lot better! Thanks a lot for looking at it :)