Store the metadate “last-modified” for a track and an album and add an option sort by it.
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.
