I18n: Update translation ca (100%).
I18n: Update translation es (100%).
I18n: Update translation ru (100%).
I18n: Update translation et (55%).
Refactor command button.
Improve build system.
Rename Menu class to Window.
Rename PanelPlugin class to Plugin.
Rearrange files to match standard layout of Xfce plugins.
Change license notice style.
Improve handling of translations.
I18n: Update translation fr (100%).
I18n: Add translation zh_TW (18%).