PolySelect for Lasso Tool

[MorrowShore]

This feature involves using lasso selection tool with line mode.

In GIMP, it is the default behavior for lasso tool to not complete when released to facilitate line mode; it only completes when meeting the beginning, or pressing enter.

However, this cannot work for mobile users.

Ideally, with lasso tool it would be possible to select with lines using shift or alt key just like #1342 (and other applications),
AND there would be a separate button to summon the "poly select" style lasso tool for mobile users (like CSP).

This covers both usecases but unfortunately bloats us by 1 button (which can be mitigated by the feature that involves holding a tool button for 3 seconds to spawn their alternate mode).

VID_20241107_195920.mp4