Fix T37170: Missing curve outline when using constructive modifiers

Issue was caused by wrong bool flag assignment.
Bring back edit curve add menu

It was wrongly considered unused in rB5fdfa6d47542db32426ac080f071a1fd751e05a7
Fix T37056: Making segment flips curves it's not needed

Made the system around splines order a bit smarter, so
crating a segment between two splines wouldn't switch
direction if splines are selected in a way that they're
"co-linear".

It is possible to make things even smarter using active
point and so, but that i'd consider a TODO.