#19 - Better support for Mobile Chrome

This change allows one to connect in a pretty straightforward way to a Mobile Chrome running on an Android device attached to the PC via USB:
* No Android SDK installation required (the ADB tool is directly embedded inside SDBG now)
* No manual fiddling with ADB start/shutdown, port forwarding etc - ADB is silently setup and teared down by the SDBG plugin
* Multiple simultaneously-attached Android devices supported, user can either visually choose amongst them or specify a Device ID pattern in the Mobile Chrome Launch Configuration dialog

The change looks like a big one because I took the time to rework BrowserManager so that it serves all launch configurations. In the original Dart code the chrome-app launch configuration was not really using BrowserManager, but had a separate code copied from it and altered a bit, as BrowserManager was not really flexible enough to support the Chrome-app launching use case as well.