Added an AboutScreen Property

Added an AboutScreen property for info that will be shown in the
system menu. Created a new property editor, TextArea, for long texts
Clarified the Notifier documentation for ShowChooseDialog and
ShowMessageDialog

Change-Id: I08efe7db25d715e1728fcdca9cae5795192c0214
Added more <users-features> to the Companion

This should result in the Google Play Store showing the Companion to
Tablet users.

Change-Id: I8eb2db1786adce86d61885c39cb881013f3dc419
Properly Handle Reading Larger Files from GCS

The Google Documentation shows reading GCS files with one read() call
to an open file channel. However on App Engine the read() call may
return fewer bytes then the file contains in which case you have to
call it multiple times until all data is read. Assuming this change
fixes the problem with have with files larger then 8MB, we will submit
a bug report for the GCS/App Engine documentation.

Change-Id: I58d982d0f36932121d5ccb91d0400aabb55c1536
Enhanced Companion: Can now scan and load APK files.

When you package an app and generate a QR Code, you can now use the
Companion to scan that code and it will call the package
installer. This means that you no longer need an external bar code
scanner to install packaged files from App Inventor. The Companion
provides you all that you need!

Also fixed #520. If you start the Companion when you don’t have a WiFi
connection, you will receive an Error Dialog box after which the
Companion will exit. No WiFi connection is needed for a USB
connection.

Change-Id: Ia0d96b1a7e49fd6e3fc5801ad86c9124adbe0a71