enhance addTextBox functionality
enhance addTextBox functionality

-textbox contol can have a HTML prepending content
-textbox contol can have a placeholder value for empty textbox


Former-commit-id: 1044ea90bd1092d693313a3d4009dcbca1627e9c
webgme/webgme-engine,webgme/webgme-engine,webgme/webgme-engine