Integrate features from metafacture-ide

[fsteeg]

In the metafacture-ide Xtext project, we have some Flux UI features: https://github.com/culturegraph/metafacture-ide/tree/master/bundles/org.culturegraph.mf.ide.ui/src/org/culturegraph/mf/ide/ui

Some of these use implementation code in: https://github.com/culturegraph/metafacture-ide/tree/master/bundles/org.culturegraph.mf.ide/src/org/culturegraph/mf/ide

Two features are very useful for working with Flux:

• [ ] https://github.com/metafacture/metafacture-flux/issues/6: Content assist with available Flux commands + documentation: doc, ui, impl
• [ ] Semantic/type-based validation of the Flux workflow structure/order: doc, impl

These are probably more nice-to-have:

• [ ] Outline view customization: doc, ui
• [ ] Launching support: doc, ui, impl
• [ ] Workflow visualization: doc, impl, based on an Xtend template and Graphviz. In a playground app (see https://github.com/metafacture/metafacture-fix/issues/19), a graph visualization based on vis.js like in https://github.com/hbz/lobid/issues/427#issuecomment-743080779 could be easily created based on the Xtend template

We should create individual issues for these (some, like the visualization, might rather go into the new playground repo, some, like launching, into both). Will add links above as we proceed.