Draft: [Corners LPE] Cherrypick

Backport of my 7 corners mr.

waiting code LAND in GTK4 master.

Legend	MR	Merge Order
Ignore segment of 2	!6231	-
Recalculate corners on changes	!6251	-
Alert changes	!6252 (merged)	0 (2023-03-23)
Knot fixes	!6253	-
Retain on joins	!6254	-
Stroring improvements	!6255	-
Fix corners joins on separate items	!6256	-

Fix issue joining 2 segments when corners LPE. Is not full fixed because one of them lose its fillets. Anyway also fixes:

• Issue updating corners after combine.
• Fix a issue knots are not updated postion on undo.
• Fixes retain corners on joins

Fixes (some of them maybe is fixed on master already):

#4599
#4447
#4736
inbox#10100 (improve)