Checks dropdown may fail to display checks

[julen]

Steps to reproduce:

1. Open the editor in a context where there are no failing checks, but trigger the checks UI with .../translate/#filter=checks. At this point no failing checks are displayed.
2. Change the filter to something else, e.g. All
3. Perform a translation which triggers any type of check failure.
4. Select Checks from the filtering options.

Observed:
The failing check is not displayed in the drop-down.

Expected:
The failing check is displayed in the drop-down.

Note this was also the case before (as already commented in #80 (comment)) but now it becomes a bit more prominent as selecting Checks allows to browse through all checks, and the UI expands too.

The problem is that elements are removed from the DOM (motivated because hiding option elements doesn't work reliably across browsers), so after that event they won't be available again until a full page refresh. Ideally we should be able to declaratively specify the check groups and checks we want to display, and don't worry about the DOM at all.