Change the header bar title when selecting items to import
This changes the header title when selecting items to import as recommended in the UX review in #89. Instead of showing the name of the device followed the number of selected items in parentheses (see the following screenshot), it will be:
- "Select items for import", when nothing is selected.
- "Select items for import (%s)", when one or more items are selected. The %s will be replaced with the appropriate plural form of "%u selected".