Support Shift-Enter for new lines in composing messages

[rexbron]

Problem:
Pressing Shift-Enter sends the message

Expected Behaviour:

Pressing Shift-Enter should create a new line.

[viktorstrate]

I think we should consider rewriting the input field to use NSTextView. That way we can have full control over multiline, custom formatting, and autocompletion of mentions.

[viktorstrate]

Added in #86