Add guard for PropertiesObject some-boxed-struct property test
Refactor signal and property finders by introducing a NullBuilder
Add guard for BoxedStruct string_ field test
Clean up finder method creation
Use finder builder to build existing find_method methods
Implement assignment to struct fields

Makes struct classes descend from a new base class, StructBase, which is
a DataConverter. Due to this, struct classes are now their own FFI type.
This means they can be used as-is in a layout specification, and no
longer cause a type mismatch during assignment.

The test was adjusted to take into account that this is a value
assignment, not the assignment of a pointer.
Add test for GIMarshallingTests::NotSimpleStruct's pointer field
Add tests for some-boxed-glist property accessor methods
Add tests for PropertiesObject's some-boxed-struct property
Make #get_property work with gchar properties
Make #set_property work with gchar properties
Add tests for some_char property accessors
Add tests for PropertiesObject's some-double property
Add tests for PropertiesObject's some-float property
Add tests for PropertiesObject's some-int and some-int64 properties
Handle properties of type long
Add tests for PropertiesObject's some-object property
Handle properties of type strv
Handle properties of type uchar
Alphabetize test order