Replace the build system “setuptools” with “meson”.
Fix both the icon of the volume button and setting the volume if the mixer does not allow it by using -1 as default value instead of 0.
Upate all GTK UI files by saving them with Glade 3.38.
Use GTK Composite Templates for GUI elements to clean up and simplify the code for widgets and all UI elements. This includes splitting the large “gtk.glade” file into smaller .ui files and the large “widgets.py” file into smaller .py files.
