Launch web app manually, instead of using the webapp command line argument.

This makes it more resilient to killing Gecko when it is a background service.
Added TODOs about icons and notifications during install.
Too clever by half. Fixed the missing splash screen.
Spurious bug fix found during uninstall.
Request ID and installOrigin preserved across Java/js barrier.

This allows getInstalled() to work properly, the DOMRequests to correctly respond, and install_allowable_from to correctly work.