Support skipping stages in scripted pipelines for nice visualization in blue ocean and classic UI stage view

Created as response to comments of https://issues.jenkins-ci.org/browse/JENKINS-37781 as the visualization of skipped stages is very, very nice for declarative pipelines in Blue Ocean.

1. Allow to skip stages in scripted pipelines leading to equally nice visualization in Blue Ocean: The current approaches mentioned by mkobit in https://issues.jenkins-ci.org/browse/JENKINS-37781?focusedCommentId=294965&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-294965 and jamesdumay in https://issues.jenkins-ci.org/browse/JENKINS-37781?focusedCommentId=294966&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-294966 lead to either misleading or just less obvious visualization...
2. Improve visualization of stage view: instead of showing skipped stages (declarative pipelines) as always being green and allegedly executed, make them e.g. gray.
   • Stage "skipped" is actually skipped, but stage view shows:
   • This is IMHO also a major enhancement and valid fix for the other problem reported by mkobit in https://issues.jenkins-ci.org/browse/JENKINS-37781?focusedCommentId=294965&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-294965: "[The Pipeline Stage View Plugin] has some weird display issues if new pipelines have different stage executions than previous ones."