rename underscored functions

[joequant]

This renames _g_object_unref to g_object_unref and _g_file_equal to g_file_equal

[mtwebster]

I wonder if it's better to use g_clear_object(&foo), as _g_object_unref behaves more like that, at least in that it checks for NULL first.

The same with _g_file_equal - it's NULL-safe.

It might not ever be an issue where you're changing it - if you're certain about it I don't have any problem merging this.

[clefebvre]

I'll close this. gThumb uses its own g functions, there are hundred of calls to it all over the project. We probably want to keep rebasing going forward so there's no need to introduce unnecessary changes.