Fix empty histograms when adding a new Filter Layer

Stuffins Lastnamerequested to merge
stuffins/krita:stuffins/fix-filter-layer-empty-histogram into master

Adding a waitForDone call fixes the empty histogram bug (BUG:506108) when adding a Filter Layer (affects Levels, Threshold, Color Adjustment, and Cross-channel color adjustment). This would be a partial revert of this change that removed the refreshGraph() call just before creating the previewDevice. In master, previewDevice is always empty.

Test Plan

Add a new Filter Layer, check that the histogram in the initial dialog works for the Levels filter.

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.
  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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