Added compatibility to Thunderbird 15.*

Added Compatibility with Thunderbird 13.*

Changelog:
- Compatibility update with Thunderbird 12.*
- I modified the display of read/unread message.
- I added oxygen theme for the WebMail Notifier addons.

Changelog:
- I correct the missing icons from the smilies drop-down menu when composing a message.
- I correct issues related to the Lighning calendar extension.
- I correct issue related to selecting text in the e-mail address field
- I correct issue related to checkboxes.

- It is a major update of the precedent version which was a little experimental.
- I' have changed all the icons by Oxygen ones.
- It has now a better support of Lightning Extension.
- I have cleaned the CSS and remove unused files.