add option to the MouseWheel handler to trigger up/down events only when wheel is released

[ahocevar]

Currently, wheel-zooming a map causes lots of tile requests to be sent to the server. Although these requests are aborted when zooming to a new zoom level, this causes a huge server load for servers that receive the abort during rendering.

The proposed patch adds an interval option to the MouseWheel handler. When set to a number (milliseconds) other than 0, the handler will cumulate the wheel delta until the wheel is released, and then zoom to the according zoom level when the interval is expired.

Tests pass and example works in FF3 and IE8. Thanks for any review.

[elemoine]

Patch looks good to me. Please commit.

[ahocevar]

(In [9799]) added option to the MouseWheel handler to trigger up/down events only when wheel is released. r=elemoine (closes #2345)