Fix spinbox decimal issues when update_on_text_changed = true

[ryevdokimov]

Fixes: #100620

When using update_on_text_changed:

If . is entered as the first character, automatically change to 0.
Text ending in . does not get submitted, unless "accepted".
Round numbers do not automatically have a decimal appended to it, unless "accepted".

2024-12-20.18-53-49.mp4

[KoBeWi]

, is also supported in SpinBox, so you should probably account for that too.

[ryevdokimov]

, is also supported in SpinBox, so you should probably account for that too.

Done.

[KoBeWi]

When the number ends with period and you press Enter, the period will disappear resulting in wrong text. Pressing Enter again will update it correctly.

godot.windows.editor.dev.x86_64_U2TJsmHcSd.mp4

[ryevdokimov]

When the number ends with period and you press Enter, the period will disappear resulting in wrong text. Pressing Enter again will update it correctly.

Fixed.

[KoBeWi]

The initial value is missing trailing 0 for some reason.

Expected:

[ryevdokimov]

The initial value is missing trailing 0 for some reason.

Fixed.

[KoBeWi]

Works correctly now.

[Repiteo]

Thanks!

[akien-mga]

Cherry-picked for 4.4.1.