Allow activeTarget to refuse to surrender

This affords some more flexibility in manually setting the activeTarget
without worrying about disrupting the current activeTarget situation;
the activeTarget can refuse if it's undesirable.
Merge pull request #1236 from mczepiel/active-target

Allow activeTarget to refuse to surrender
Update mock dom to share EventTarget methods
Unload the overlay press composer when it isn't shown

Because it calls preventDefault even when the overlay isn't shown,
which breaks non-pressComposer/native interactions
Merge pull request #1248 from Stuk/overlay-click

Overlay press composer unload
URL.resolve should not cache the document's base element.
URL.resolve would fail if the document's base node is set or replaced after montage has been initialized