Reorg dirs: rename some files

t.knopffrequested to merge
reorg_dirs into develop

Relates #106 (closed)

In this MR some files were renamed to avoid names that contain "Error" or "Warning", i.e.

  • ProjectLoadWarningDialog was renamed to ProjectLoadProblemDialog
  • WarningSign and WarningSignWidget were renamed to CautionSign and CautionSignWidget (which seems okay, because the sign is yellow, compare with here)
  • in preparation some unnecessary includes were removed

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