[GTK][Fix] Call create_file_dialog from main thread
[GTK][Fix] Call create_file_dialog from main thread

Also refactored idle_add calls

r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,shivaprsdv/pywebview,shivaprsdv/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,r0x0r/pywebview