Fix GzSpinBox in cases where the value property has a binding

[azeey]

🦟 Bug fix

Summary

Before this fix, if the value property has a binding instead of a value,
any modification from the GUI (e.g. hitting the up arrow on the spin
box) would not actually change the displayed value because it is
immediately reset back to the value set by the binding. This is the case
in the GzPose module and the effect can be easily be seen in the
GridConfig plugin. To test, run:

gz gui -c examples/config/grid_config.config

You will see that, before this change, modifying the x,y,z,roll,pitch,
yaw values actually changes the grid configuration, but the updated values are
not displayed in the spinbox.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.