Fix #148 Improve css styling and fix theming issues

[tortmayr]

• Remove Thememanger for now and use one style for both Theia Themes
• Refactor css stylesheets (importing of base stylesheets, removing redundant code)
• Fix the WorkflowPopupModelFactory (server-side) which was not working anymore after [Workflow-Example] Operation.elementTypeId does not match actual elementTypeId #143
• Minor code clean up (Resolving formatting warnings, remove unused files)

[planger]

Cool, thanks a lot!

[tortmayr]

Thanks for the review. I will fix the conflicts.
While we are at it: I noticed that all code reviews get dismissed when new code is pushed to the PR branch. This is also the case for already approved changes. As a consequence we always have to ask for a new review after resolving conflicts or other minor fixes.

[tortmayr]

Apparently this can be disabled (isaacs/github/issues/1157) in:
Settings ▶ Branches ▶ Branch protection rules ▶ Edit ▶ Uncheck "Dismiss stale pull request approvals when new commits are pushed".

[planger]

LGTM! Thanks!