Better integration of pdfannots library

[tgraupne]

Currently, the flet GUI is only using Python's subprocess.run() to utilise the pdfannots library.

subprocess.run(["pdfannots", radio_group.value], capture_output=True)

As far as I can tell, this is causing at least the following issues:

1. I believe app users need to install the pdfannots independently. A first test on another MacOS computer with a Mac App bundled with PyInstaller according to the flet Getting Started Guide did not work.
2. Using all possible options and parameters of pdfannots is very cumbersome.

Hence, I was wondering if the community could help me integrating pdfannots on a library-level?

I would be especially grateful if maybe @0xabu or maybe @IgorAvilaPereira would be interested in talking about this. Since the topic of building a GUI for pdfannots has already been discussed in the projects GitHub repo: 0xabu/pdfannots#64