Improve addFont method for DefaultFont in Document class

[microshine]

The current implementation of the addFont method in the PDFDocument class for DefaultFont allows registering the same font multiple times. This behavior needs to be improved so that the method can detect if the added font is already registered and return the existing font instead of registering it again. Additionally, when registering fonts, a random UUID is assigned to each font. For system fonts, it would be better to use special names instead of UUIDs (e.g., Acrobat registers Helvetica as Helv).

Steps to reproduce:

1. Create a PDFDocument instance.
2. Call the addFont method multiple times for the same DefaultFont with different names.
3. Observe that the font is registered multiple times.

Expected behavior:

The addFont method should detect if the added font is already registered and return the existing font instead of registering it again.

Actual behavior:

The addFont method registers the same font multiple times.