Preference pages for props and yaml ls editor

Allows configuring problem severities.
The ui works but is not yet wired to transmit
these setting to the language server.

eclipse-extensions/org.springframework.ide.eclipse.editor.support/META-INF/MANIFEST.MF

@@ -25,7 +25,8 @@ Require-Bundle: org.eclipse.ui,
  org.reactivestreams.reactive-streams;bundle-version="1.0.0",
  org.apache.commons.lang3,
  org.springsource.ide.eclipse.commons.ui,
- javax.inject
+ javax.inject,
+ com.google.gson
 Bundle-ActivationPolicy: lazy
 Export-Package: org.springframework.ide.eclipse.editor.support,
  org.springframework.ide.eclipse.editor.support.completions,