placeholders and empty states

[oliver-sanders]

Stand-in content for empty / null states to make users aware of the state (rather than just displaying a blank page).

Proposal to close the empty/loading components of #111

Modern UI design seems to have settled on a greyscale background image for empty states these days. Clear, concise and simple to implement.

I believe material design has built-in support.

Some examples of places where we need empty state placeholders and some ideas as to what they could look like:

Missing placeholders:

• [ ] Workflow: workflow stopped
• [ ] Workflow: workflow does not exist (e.g. workflow deleted)
• [ ] All view components: connecting/loading (spinner / skeleton)
• [ ] All workflow components: no tasks matching filter
• [ ] Workflow: network error (unable to get data)
• [ ] Workflow: no active view
• [ ] GScan: no workflow
• [ ] GScan: no workflow matching filter

Pull requests welcome!