Spring Boot configuration property auto-completion does not offer properties on super classes

[yl-yue]

Details of specific issues #186

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Sorry！Please excuse my laziness

[martinlippert]

@kdvolder can you have a look?

[kdvolder]

I think we have ticket for this already, but it is in old spring ide repo: spring-attic/spring-ide#315

[kdvolder]

We should probably close one of two tickets as a duplicate. Perhaps its most logical to keep this one because it has to be fixed in the boot language server which is a component of STS 4.

[kdvolder]

Note: this applies to both .yml and .properties files. So a fix should make sure it works in both contexts.