[packagechooser] Introduce translatable labels

- Since the package chooser might be used more than once, or for
   more specific items than "Packages", introduce a way to provide
   specific strings for display.
 - The only string needed is the ViewStep name, since the item with
   id "" can be used for the no-selection item.

src/modules/packagechooser/packagechooser.conf

@@ -20,6 +20,19 @@
 # or one-or-more).
 mode: required
 
+# Human-visible strings in this module. These are all optional.
+# The following translated keys are used:
+#  - *step*, used in the overall progress view (left-hand pane)
+#
+# Each key kan have a [locale] added to it, which is used as
+# the translated string for that locale. For the strings
+# associated with the "no-selection" item, see *items*, below
+# with the explicit id "".
+#
+labels:
+    step: "Packages"
+    step[nl]: "Pakketten"
+
 # Items to display in the chooser. In general, this should be a
 # pretty short list to avoid overwhelming the UI. This is a list
 # of objects, and the items are displayed in list order.