Save options

[jcupitt]

This is @angstyloop's PR #15, with a few changes:

• It uses the GtkDialog framework to show the dialog window, so SaveOptions is a true widget, and a subclass of GtkDialog. This saves quite a bit of code, and make it behave more like a regular gtk dialog (so the popup is draggable, for example).
• The dialog (though not all the contents, of course) are defined in a .ui file, reducing code a bit more.
• It looks like a regular dialog to callers, saving some code there too.
• I simplified the widget tree somewhat.
• angstyloop's nice introspection code for creating and reading the option widgets is still there.

To do:

• [x] An error and progress bar in the save popup.
• [x] delayed return of the accept response, for long-running saves.
• [x] a couple of small memleaks, with FIXMEs on them
• [x] the filechooser is being destroyed too early, causing a warning
• [x] cleanup and remove most dbg code

[jcupitt]

I think this is all done now.

[angstyloop]

Wow! Thanks J!

[angstyloop]

Ha ha

[jcupitt]

OK, let's merge!

[jcupitt]

I made a v2.5 release with this improvement. It should be on flathub shortly.