Detect missing attachment on send

Before sending an email, the message editor should check the text of subject and message for the presence of text "attach,attached,attachment,...". If it is present, it should check for the presence of attachments in the attachments list. If none is present, it will warn the user, and ask if he/she wants to send anyway.

As a first step, this should work based on the user language and english by default.
This means that we need a resource containing a list of patterns to be searched, for example:

editor.detect.attach.patterns=attached,attachment
or
editor.detect.attach.patterns=allegat

etc.

As a second step we will introduce a library to detect the language of the text and look for the correct patterns of the detected language.