v4.0.0-alpha3

Project optimizations:

1. Update GUI framework to Qt6 (plugin feature will disable in Qt5 when 4.0.0 stable release released)
2. Delete some building platforms from Github Action

Bug Fixes:

1. Fix encoding error in Qt6
2. Fix the choosing error when finished download a theme

Enhancement:

1. Expand the background icon to 6144x6144
2. Optimize cppparser

New Features:

1. Add the plugin system in software.

Full Changelog: v3.4.0...v4.0.0-alpha3