Show alternative images for states "Initialization" and "Take Image"

[Slider0007]

The Feature

Follow up to this discussion #1809 about alternative images for the state "Flow not started" and state "Take image" due to the fact that in these states right now no, black or only partly rendered image is shown.

#########################
Providing an alternative image during "Take Image" step would be an option, also to avoid the black screen at the beginning of the process.

What is also missing is the visualization when flow is not started yet. Also for this we could show an indication image which is loaded from SD.

Possible steps:

1. Device / webserver started, but flow not started (yet) --> Image with indication "flow not started"
2. Flow started and step "Take image" --> Image with indication of taking image
3. Flow started and step "Alignemnt" --> serve alg (like already coded + ?alignment icon?)
4. and the follwoing steps --> serve alg_roi (like already coded)

Originally posted by @Slider0007 in #1799 (comment)

[Slider0007]

Implemented with #1858