pdftag 2.0.0

Port to GTK4 and libadwaita:
- redesigned UI with boxed lists, header bar actions and toasts
- primary menu with keyboard shortcuts and about dialogs
- proper GApplication (relative paths and URIs work on the CLI)
- atomic saves that preserve file permissions, run in a worker thread
- keyboard shortcuts: Ctrl+O open, Ctrl+S tag, Ctrl+Q quit
- flatpak manifest updated to the GNOME 49 runtime

fix #1 again by not hardcoding the path

First (and probably last MS Windows Version)

Add button to open PDF with default viewer

fix segfault when no file was selected

allow relative paths on the command line

first stable feature-complete version

use place_holder_text for entries