Fix syntax highlighting for GTK4
Fixes bug when selecting Syntax Highlighting in advanced export due to missing function dup_string
Traceback (most recent call last):
File "/app/lib/python3.10/site-packages/apostrophe/export_dialog.py", line 315, in export
self.retrieve_args()
File "/app/lib/python3.10/site-packages/apostrophe/export_dialog.py", line 399, in retrieve_args
selected_style = self.cmb_syntax_highlighting.get_selected_item().dup_string()
AttributeError: 'StringObject' object has no attribute 'dup_string'. Did you mean: 'get_string'?
This was introduced by the switch to GTK4, !253 (merged).