Fix AudioStreamInteractiveEditorPlugin GUI bugs

[caimantilla]

Adds DPI scaling to the table columns and fixes an error which occurs when clicking the top-right "From / To" cell.

Before:

After:

[Calinou]

If this line is never supposed to be reached, it might be better to use ERR_FAIL_COND() instead so an error can be printed.

[caimantilla]

If this line is never supposed to be reached, it might be better to use ERR_FAIL_COND() instead so an error can be printed.

It does get reached though, hence why I added it.
It's been a bit since I made this PR, so I'll see if it would instead make sense to always add that metadata.

[caimantilla]

If this line is never supposed to be reached, it might be better to use ERR_FAIL_COND() instead so an error can be printed.

It does get reached though, hence why I added it. It's been a bit since I made this PR, so I'll see if it would instead make sense to always add that metadata.

Now I remember, "to" is an integer referencing a clip. It's not written to the corner cells of the tree (ie. the "From / To" header)
It doesn't make any sense to write it there. Still, checking if "to" exists is a bit specific, so instead I've made it so that each cell has an "is_editable_cell" metadata and check that.