Add subvariations to Milner-Barry Gambit and King's Gambit Declined

[seccohazy]

Sub-variations and fixes in the Milner-Barry Gambit:

Added "Hector Variation" — By now probably the accepted name for 7. O-O. As seen in Coffeehouse Repertoire Vol. 2 (2021), for example.

Added "Main Line" — makes sense now to distinguish the traditional main line, but also has the benefit of 'catching' the Milner-Barry Gambit when it emerges from the Advance's "Wade Variation" and "Euwe Variation" move orders.

"Sorensen Variation" — here I only replaced the Danish "ø", primarily to accommodate lichess's search function.

Added "Walthoffen Attack" in the King's Gambit

Source: "The King’s Gambit" feature article in Edward Winter's wonderful Chess Notes.

[niklasf]

Thanks!

"Sorensen Variation" — here I only replaced the Danish "ø", primarily to accommodate lichess's search function.

Reverted this part. We should fix this in search, instead.