Load gettext on startup

Initialize gettext of C library and Python gettext module.
Set the translation domain of GtkBuilder files
Remove uses of Pango markups in data/AlbumWidget.ui
Mark some of the strings as translatable
Add back updated translations
view: Set static properties of renderers in Songs view only once

We repeatedly set the static properties of renderers, such as xalign,
everytime the data function of renderers are called. We could just set them
during construction.
view: Set icon name for star renderer of Songs view only once

Since this value do not change, we could just set this during construction
then map the visibility of the renderer to the list store column storing
whether it should be visible or not.
view: Store the media title and artist during populate on Songs view

Load media title and artist to the list store then map them to "text"
attributes of their renderers.
Merge pull request #117 from kyoushuu/i18n

Initial work on internationalization
Merge pull request #119 from kyoushuu/simplify-songs-view-renderers

Simplify Songs view renderers
view: Mark "All Artists" as translatable
Remove some "translatable" attributes in GtkBuilder files
Merge pull request #121 from kyoushuu/i18n-2

More internationalization fixes
fix to #44 and cleanup.