15270 Unclear if filter is expanded

[``* `#15270` Unclear filter expand state``](https://www.openproject.org/work_packages/15270)
@hschink works for me, but I'm not sure it's a good idea to concatenate translation strings (see [this article for why](http://www.smashingmagazine.com/2012/07/18/12-commandments-software-localization))