Rework AcceptDialog's ok button text

[KoBeWi]

AcceptDialog's ok_button_text was directly operating on the button's text. This might be ok if the text does not change, but it does change. FileDialog was overriding it, making it not editable and causing a translated text to be saved in the scene file.

This PR reworks how the text is used. It's split into 2 properties: one is the user-defined ok text, the other, internal, property is the default ok text. When the user's ok text is empty, the internal text will be used. FileDialog uses now internal text.

godot.windows.editor.dev.x86_64_FpzxQRkYEv.mp4

Fixes #79803
Fixes #62827 (comment)
Supersedes #79848

This slightly breaks compatibility. Dialogs that previously had ok text defined will not be able to override it internally (affects only FileDialog). Also you can't set empty button text; at best you can put a space.

[thatsmaik]

I just stumbled across this issue on my project myself. Took me a while to understand that something internally doesn't seem to work properly. Please review this and push it into the main branch if possible - right now this FileDialog property causes confusion and should work as described by KoBeWi.

The use case for me to change the OK button text with the Open File mode was to load a custom project save file and i would like to rename the OPEN button to LOAD to underscore what is going on. The window title does the job, sure, but they work best in tandem in my opinion.

[Calinou]

Tested locally, it works as expected. Code looks good to me.

Given the compatibility breakage, it would be best to merge this early in the 4.5 cycle so it's in 4.5.dev1.