GSoC 2020: Enable live preview of Fill Layers

This MR enables live previewing Fill Layers before confirming addition.

So far, this MR doesn't work -- the jobs have been successfully migrated, but they do not execute at all.

I'm grateful if anyone could have a look at it and help me go, one step at a time.

Plan

• Remove use of KisGeneratorStrokeStrategy
• Decouple update job generation from the delayed update/signal compressor timeout
• Connect the Fill Layer generator dialog to the stroke and send the jobs over to the stroke

Test Plan

Add a Fill Layer, check it's instantly rendered.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

cc @dkazakov