Give a max width to plain text email composer

Set a maximum width in the composer screen for for plain text emails, to communicate to the user that Geary automatically wraps long lines. Closes #535.

One caveat is that we just rely on the default CSS text wrapping to communicate that some kind of text wrapping happens. We don't actually implement the format=flowed line-wrapping algorithm, which is what Geary uses when it sends an email. It think it is important that if the user switches from Rich Text to Plain Text and then back to Rich Text then the text should go back to it's original state. Therefore, we probably don't want to have the Plain Text composer changing where the line breaks are in the underlying text.