Fix editData in HTML/UWP extensions allocating too much data

[AITYunivers]

editData is allocated using
(in v1)

this.sizeBytes - header.byteLength

(in v2)

this.sizeBytes -
// skip area between eHeader -> Props
(ext.ho.privateData - 20) -
// Skip DarkEdif header
header.byteLength

neither of these equations account for the size of eHeader, and in v2 the header not including verStr in it's length.

This PR fixes these by subtracting eHeader and by subtracting 4 (in the Template only as the extensions are v1)