Fix Spline tool in-progress drawing being cleared on tool change

[LixiviaDev]

Spline Tool: Spline won't stay unless closed or (Esc or Enter) is pressed.

My changes:

• Removed "::AbortTransaction" message, which completely cleared the new spline on "::abort".
• Added "delete_preview()" function to delete the preview point that tracks the mouse.
• Added "::EndTransaction" message to signal that we want to save the new spline as is.

[github-actions[bot]]

📦 Build Complete for 1c24ad09281adc0c5085633036edb8d197dbae8e
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[Keavon]

This does not appear to solve the problem in the specific case of switching to the Select tool (with the V hotkey or clicking its icon). It does seem to work as expected when switching to the other tools that I've tested. Could you please take a look at solving it for that tool?

[LixiviaDev]

I'm really busy these days, I'll take a look at it as soon as I can