Fix an issue with Chrome and hidden tabs.

[manthey]

When the webpage is on a hidden tab, tiles are not rendered. This seems to only be a problem with Chrome. This adds 'hidden' and 'unhidden' events, so any feature can hook into these.

The webgl tile layer refreshes its tile cache when unhidden.

[manthey]

This doesn't have automatic test coverage; we'd have to add quite a bit of code to have Chrome with multiple tabs and code coverage.

You can test this manually -- on master, load the tile example. One a developer console as a separate window. Switch to another tab. In the developer console, hit reload. Wait ~10 seconds for the hidden tab to finish loading tiles. When you switch back to the tab with the tile example, the map will be blank.

It is a bit tougher to show off the corrected code without building locally -- the tutorial can't be used since it gets refreshed properly after being hidden since it is in an iframe.

This is a problem seen in actual deployments -- open a tab and switch away before the map finishes loading; switching back and those tiles that were loaded while hidden will be blank.