Change Gio::Action 'transform-translate' to use a tuple Glib::Variant (dd).

One problem with the current Gio::Action setup is that complex arguments are given as strings. For example, the argument for 'transform-translate' requires two doubles, but we read in a string and then try to parse it in the function called by the action.

It would be better if we utilize Glib::Variants to specify the exact format expected for complex arguments. In the case of 'transform-translate' this would be "(d, d)", where the parentheses indicate a tuple and the 'd's indicate doubles.

See https://developer.gnome.org/glib/stable/gvariant-format-strings.html for how to describe arbitrary formats.

Not part of this merge request:

We currently use a syntax where actions are separated by ';' and arguments are separated from action names by ':'. We might consider changing the latter to use the syntax used by "detailed actions names", which uses parentheses to enclose the argument, e.g. 'transform-translate((50.0, 25.0))' (the outer parentheses enclose the argument while the inner ones indicate a tuple). This would allow us to directly parse the "detailed action name" into the action name plus a Glib::Variant of the matching type using Gio::Action::parse_detailed_name_variant(). One problem with parsing "detailed action names" via this function is that the Glib::Variant created is not always of the correct type. For example 'transform-translate((50,25))' is parsed as a tuple of two int's (i, i). We would have to check the parsed type against the expected type and then make modifications if necessary (something not hard to do).