Fix TreeItem button handling

[timothyqiu]

Fixes #79629

This PR mainly fixes get_tooltip() and get_button_id_at_position():

• They did not work when the control is too narrow and when RTL layout is used.
• They did not take button_margin, h_separation, and item_margin into account.

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TestProject.zip

[akien-mga]

Would it make sense to do the revert part only for 4.1 and 4.2, while the rest of the more involved fix gets assessed for 4.3?
Or is the regression it caused less bad than the bug it worked around?

[timothyqiu]

Would it make sense to do the revert part only for 4.1 and 4.2, while the rest of the more involved fix gets assessed for 4.3?

Ah yeah, the bug it worked around only exists if you make a tree too narrow, adjusting window size solves the problem. But the regression it caused has no workaround, many tree buttons in the editor lost their tooltips.

[akien-mga]

Could you make a revert PR for 4.2 with cherrypick:4.1 explaining the rationale?

[akien-mga]

Or well actually, maybe do a revert PR for master to merge first and cherry-pick (reopening the original issue), and then rebase this one on top.

[timothyqiu]

OK, done. #90842.

I'll rebase this one after it's merged.

[akien-mga]

Thanks!