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
Merge branch 'master' of https://github.com/mit-cml/appinventor-sources