Show pattern-candidates for a PatternLanguage

[manuwei]

Currently, pattern languages and pattern candidates are listed separately. However, as a reader I may not only be interested in patterns of a particular pattern language but also in pattern candidates that will likely become patterns in the future.

To further support this, display all pattern candidates below the patterns of the pattern language (if the feature toggle for pattern candidates is activated). They should be easily differenciable from patterns

[TYueksel]

I added the pattern candidates to their pattern languages by creating a new tab group with the tabs "patterns" and "candidates" (check PR. This view will only be visible if the feature toggle for pattern candidates is activated, otherwise it still looks the same as previously.