quick-settings-box: Support RTL direction

Closes: #1154 (closed)

I don't know how to properly test it, so I just forced QSB to do RTL by setting:

gtk_widget_set_default_direction (GTK_TEXT_DIR_RTL);

immediately after gtk_init in the tools/quick-settings-box-standalone.c and src/main.c.

From my testing it works good, but it will be nice if someone who uses real RTL locale can test it.

CC: @iramosu