Highlight the 'Categories' header on the detail page to make it more visible

Change-Id: I900447c2d3cbf87dc912925b1e3b8ad185f4b2c6
Tweak category list in media detail overlay.

* highlight the label
* localize label
* Add 'loading' and 'none selected' placeholders in categories list.

Change-Id: I7ea4ff93ba67fab6fa949afb53aefc434d661905