razorqt-desktop: Unify plugins and widgets names. Closes #447

Widgets is a more user oriented name.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Merge pull request #640 from luis-pereira/unify-desktop-plugin-widgets-name

razorqt-desktop: Unify plugins and widgets names. Closes #447