Add main page title to gettext i18n

Fix #807

The main change is the use of _() for translation the "Application Title" (as the page title) in the index.html_tmpl template.

I have not added default POT/PO files for the project as suggested in #807 because it is not complicated to create them following the instructions in the doc: http://docs.camptocamp.net/c2cgeoportal/master/integrator/internationalization.html#server

On the other hand it could be handy for people not very familiar with Gettext to have the base files with a few custom translations that they will most likely customize, such as the "Application Title". Should I proceed?