AUI-1312 text inputs within a DD node cannot receive cursor focus with a mouse

Hey @eduardolundgren

This is a fix for AUI-1312/LPS-46385 (a priority 5).

This commit has already been merged into yui3 dev-3.x branch, so I just cherry-picked it onto our patched-v3.16.0 branch. Has to do with ensuring that we don't prevent default on mousedown for non-drag handles.