Automatically fit to size all new boxes placed on diagrams

[kbirken]

From mbeddr.core created by stephaneberle9 : mbeddr/mbeddr.core#1861

Sometimes, when new boxes are placed in a diagram, their text becomes very small and is hard to read. The problem can be fixed manually by clicking the Fit to size tool botton. But it would be better, if new boxes were automatically fitted to their size as soon as they appear in the diagram