Updated screen size handling; various other tweaks

- Removed target-densitydpi and instead adjusted for
window.devicePixelRatio to ensure correct sizes across Android versions.
- Added a title to _alert messages
- Handled iOS camera cancel message
- Added FastClick in an attempt to remove excruciatingly long click
delay on iOS (@thehogfather - any better suggestions, or a way to get
rid of the need to tap twice on iOS?)