[content assist] Keys in property files are not indexed for <ctrl-click>

See https://bugs.eclipse.org/bugs/show_bug.cgi?id=571021 for the original report in the Bugzilla.

Eclipse has good support of standard resource bundles which allow navigating between Java code and keys found in properties files.

However, as soon as you use a different lookup mechanism you are lost. Keys in property files seem to be only indexed when used as resource bundles.

So the feature request is:

If you have a properties file `foo.properties` with key-value pairs like
```
my.message=This is my message
```
then ctrl-clicking on "my.message" should show all location in Java files where this particular string is used (independent of a specific lookup mechanism).

Right now I need to mark "my.message" and use the "Find" functionality to lookup that string.

(That other IDE is doing this fine.)