Fix having several "hand written" properties

[thiblahute]

We were overflowing when calculating the "generated_prop_id", let it be
negative when setting/getting a manually added property:

thread 'object_inner_methods' panicked at 'attempt to subtract with overflow', /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/ops/arith.rs:240:1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/panicking.rs:48:5
   3: <usize as core::ops::arith::Sub>::sub
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/ops/arith.rs:233:45
   4: <usize as core::ops::arith::Sub<&usize>>::sub
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/internal_macros.rs:61:17
   5: <object::obj_inner::ObjInner as glib::subclass::object::ObjectImpl>::property
             at ./tests/object.rs:56:1
   6: glib::subclass::object::property
             at /var/home/thiblahute/devel/misc/gtk-rs-core/glib/src/subclass/object.rs:80:13
   7: g_object_get_property
   8: <T as glib::object::ObjectExt>::try_property_value
             at /var/home/thiblahute/devel/misc/gtk-rs-core/glib/src/object.rs:2515:13
   9: <T as glib::object::ObjectExt>::try_property
             at /var/home/thiblahute/devel/misc/gtk-rs-core/glib/src/object.rs:2485:20
  10: <T as glib::object::ObjectExt>::property
             at /var/home/thiblahute/devel/misc/gtk-rs-core/glib/src/object.rs:2493:9
  11: object::object_inner_methods
             at ./tests/object.rs:134:16
  12: object::object_inner_methods::{{closure}}
             at ./tests/object.rs:125:1
  13: core::ops::function::FnOnce::call_once
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/ops/function.rs:248:5
  14: core::ops::function::FnOnce::call_once
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[thiblahute]

@jf2048 Could you check my last PRs please?

What is your plan with that project? I think it is already in very good shape and would definitely be worth having more people using it!