Update i18n

Created by: ederag

First attempt to update the i18n system. This was triggered by the port to gsettings #470 and the related investigation for the correct schemas l10n.

It works fine:

LANGUAGE=fr gsettings describe org.gnome.Hamster last-report-folder
Le dernier répertoire utilisé pour enregistrer un rapport

There were spurious characters issues on the command line:

D�but | Fin   | Dur�e    | Activit�     | Cat�gorie

That are fixed now:

LANGUAGE=fr hamster list
Début | Fin   | Durée    | Activité     | Catégorie

Slight developer workflow change: the scripts were renamed with a .py extension, otherwise xgettext did not recognize them as python files. No user-visible change since these scripts are installed under the same old names.

Note: do not use the uninstalled scripts to test i18n since, as previously: https://github.com/projecthamster/hamster/blob/250c559778e703910b7c295511669ee4362fbdce/src/hamster/lib/i18n.py#L14

The i18n is not effective for the overview yet, either on openSUSE or in the lubuntu-19.04 virtualbox. But that did not work before this PR anyway, so that might come from my setups... To be investigated later.