Update POTFILES & i18n support

Closes #27 (closed)

• add gettext () / _ () for translatable text in *.vala
• add i18n support in src/main.vala

Tips

There is tips to how add new translation for Black box:

1. Change work directory into this project, and run meson build, meson will create build directory for blackbox:

   cd blackbox
   meson build

2. Add languages to po/LINGUAS, for example, if we want add Simplified Chinese translation, we can add zh_CN into po/LINGUAS
3. cd into build dir, then run ninja blackbox-update-po, meson will generate / update .po files in po/ directory:

   cd build
   ninja blackbox-update-po

4. edit .po file under po/ directory.