Added french translation and '*' for files import.

[rpeyron]

Added a very basic '*' in order to import all media files extensions.

Added french translation, and made a few changes to make translations work:

• Added translations loading (and not within init action because it needs to be located before plugin registration)
• Fixed some wrong text domain names in some strings
• Added some missing translatable strings
• Added description translation (with a dummy string in order to force gettext to get the translatable string)