This commit includes a complete refactor of ModestMsgView. Now

This commit includes a complete refactor of ModestMsgView. Now
ModestMsgView is an interface, and we have ModestGtkhtmlMsgView and
ModestMozembedMsgView.
* src/modest-tny-platform-factory.c:
        * Get ModestMozembedMsgView from factory, if enabled.
* src/modest-msg-view.[ch]:
        * Now this is an interface. ModestGtkhtmlMsgView and
          ModestMozembedMsgView implement this interface. This
          way we make easier to use one or another implementation.
* Added src/widgets/modest-gtkhtml-msg-view.[ch]:
        * ModestGtkhtmlMsgView contains the old ModestMsgView widget
          (with all the gtkhtml related things). It implements the
          new ModestMsgView interface, and uses all available
          interfaces (zoomable, isearch view, etc).
        * Reorganized the code, to separate the internal code from
          interfaces implementation.
* Added src/widgets/modest-mozembed-mime-part-view.[ch]:
        * Inherits from TnyMozembedMimePartView, adding implementation
          for ModestZoomable, ModestMimePartView, and stub
          implementation of  ModestISearchView.
* Added src/widgets/modest-mozembed-msg-view.[ch]:
        * Implementation of ModestMsgView interface using mozembed
          backend. It's not finished nor really working, but it's
          a technology preview.
* src/widgets/Makefile.am, src/Makefile.am:
        * Added new widgets.
* src/maemo/modest-msg-view-window.c:
        * Removed dependency on gtkhtml code. Update access to
          ModestMsgView to use the proper interfaces.
        * Now construct does not use a msg parameter. Message is set
          after construction.
        * Update event names of ModestMsgView.
* configure.ac:
        * Add mozembed parameter, to be able to activate mozembed
          support.

pmo-trunk-r3521