Fix int and animation option writes

[soreau]

Trying to set int or animate options does not work when typing them into the spin button field. Scrolling and clicking the tick boxes to change the value does, however. Apparently, this is a problem with gtkmm when calling get_value_as_int() in the changed handler. It does not affect double options, which use get_value(). This solution uses the C API to work around the problem.

[soreau]

@NamorNiradnug I know this seems a bit convoluted, but I was hoping you might be able to help improve upon it.

[soreau]

I have been testing this and it seems to be an improvement, so merging before release.