Sigil 2.5.0 adds Linux AppImage, updates to Qt 6.8.2 and Python 3.13.2

Sigil 2.5.0 introduces a Linux AppImage build that matches the MacOS and Windows versions in features. All platforms now use Qt 6.8.2, with MacOS and Windows builds embedding Python 3.13.2.

CodeView performance is optimized, and PageEdit now syncs cursor positions with external files.

The update also brings an experimental PythonFunctionEditor, enhanced resource handling, and numerous bug fixes for improved reliability.