reducing default text size to make plots legible again after recent refactor

Release notes

Details

another minor fix after the recent plotter refactor. It appears that the overall canvas size/scaling was changed and since the size of text in ROOT plots is depending on the canvas size this causes text to be too big and frequently overlap.

This MR reduces the default value for the text size from 0.05 to 0.04 and also moves the definition of this default value into a precompiler macro (one per concerned plotter source file) to make consistent changes easier in the future.