GTK 4 centers marks of the Scale widget even when the scale has a vertical
orientation. Unfortunately, the Scale widget does not provide a way to set the
alignment or to access the internal Label widget in any way. To left-align the
labels this commit add a method that traverses the all children of the songs
scale recursively and adjusts the alignment if it is a Label widget.
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.
