Add option to change keyboard shortcuts (Shift-Enter for new line, Enter to send); consider making them fully customizable

[zandaarl]

Add keybinding for

• Register keybinding for opening Messenger window #60
• send message (maybe just add option for "Shift+Enter for new line and Enter to send" instead of the current "Enter for new line (which is default by browser in text area) and Ctrl+Enter to send)
• new line (could be done by checking _onKeyPressEvent for enter and then stop event from bubbling up)