Let right mouse button to be activation key

Adds RMB to the list of activation keys. Because TZP is usually activated by mouse, having one of the mouse buttons in activation key role is little bit more comfortable.

Tested on Windows, Firefox 15.0-32.0a1. Requires Firefox 15.0 or later. ~~It won't work on Mac OS as `MouseEvent.buttons` is not available there.~~

If you like it I could probably make it version and platform aware, so it would work for Firefox >=15.0 on Windows or Linux.

###### Checklist
 - [ ] Todos
    - [x] ~~Enable this feature for Seamonkey?~~ (too much trouble for few users)
       - [x] ~~minVersion and maxVersion can't be specified for individual apps separately~~ (~~probably~~ not an issue)
    - [x] ~~Don't prevent context menu unless TZP is really gonna display image~~ (may be difficult to determine before it does).
    - [x] ~~User may set unsupported configuration via about:config~~ (he was warned to be careful)
    - [x] ~~using deprecated way for triggering custom events for compatibility reasons. See https://developer.mozilla.org/en-US/docs/Web/Guide/Events/Creating_and_triggering_events#The_old-fashioned_way~~ (not so nasty, compatibility wins)
    - [x] Check and update TZP minVersion for Firefox. Done, see #152 
    - [ ] Lingering _preventNextContextMenuEvent() (aka "non-dragged right-click does not show the menu")
       - [ ] Mac
       - [x] Linux
 - [ ] Testing
    - [x] Windows
       - [x] Firefox 3.6 and later
       - [x] Firefox before version 3.6
    - [ ] Linux
       - [ ] Firefox 3.6
       - [x] Firefox current version
       - [x] ~~Could work with Firefox before version 15.0 as it does not depend on `MouseEvent.buttons`~~ (not worth it)
    - [ ] Mac
       - [ ] Firefox 3.6
       - [ ] Firefox current version
       - [x] ~~Could work with Firefox before version 15.0 as it does not depend on `MouseEvent.buttons`~~ (not worth it)
    - [x] Make sure these services are available (`_appInfo`, `_versionComparator`, `_runtime`) in
       - [x] Firefox before version 3.6 (Windows; probably fine for the other platforms as well)
       - [x] Seamonkey
          - [x] the oldest version ~~used by users according to stats~~
          - [x] current release
    - [x] ~~Seamonkey all versions~~
 - [ ] Addon compatibility
    - [ ] Prevent mouse events to propagate (Firegestures)?

###### References
 - ~~Reason for platform dependency: https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent.buttons~~
 - Reason for Firefox 15.0 dependency (probably just Windows): https://bugzilla.mozilla.org/show_bug.cgi?id=731878