Fixes for comments on r59655

• Look for any textbox since just creating one doesn't work
• editFormHeadInit -> showHeader
• code style
• moved checkbox initialization into relevant locations
• dropped hardcoded use of currentFocused in favor of catching event delegated focus events within the edit form

Also:

• Removed more direct uses of $wgRequest in favor of properties
• add $wgPreviewOnOpenNamespaces for extensions like SMW that have category like namespaces.