Insert emojis into subject field

[renetrefft]

If the cursor is currently placed in the subject field the addon insert an emoji at the end of the message.

The emoji should be inserted at the cursor position in the subject field instead (as plaintext emoji).

[mganss]

This should be fairly easy to implement (if the relevant APIs provide the necessary functionality). Here's the code for Outlook and here is the code for Thunderbird.

[mganss]

This has been implemented for Thunderbird in 1.6.0.