Moved tab_index to Style (instead of Field), it makes more sense there, since it pertains to the layout of stuff on the card.

Also don't precalculate list of fields sorted by tab order, just find the next/previous ones on the fly.

doc/type/field.txt

@@ -41,7 +41,6 @@ Fields are part of the [[file:style triangle]]:
 | @card list name@	[[type:string]]		field name	Alternate name to use for the card list, for example an abbreviation.
 | @card list alignment@	[[type:alignment]]	@left@		Alignment of the card list column.
 | @sort script@		[[type:script]]		 		Alternate way to sort the card list when using this column to sort the list.
-| @tab index@		[[type:int]]		@0@		Index for moving through the fields with the tab key. The default is from left to right and then top to bottom.
 
 The @type@ determines what values of this field contain:
 ! Type			Values contain								Displayed as

doc/type/style.txt

@@ -68,6 +68,7 @@ Here are some examples:
 --Properties--
 ! Property		Type					Default		Description
 | @z index@		[[type:int]]				@0@		Stacking of this box, fields with a higher @z index@ are placed on top of those with a lower index.
+| @tab index@		[[type:int]]				@0@		Index for moving through the fields with the tab key, fields with a lower tab index come first. Otherwise the order is from top to bottom and then left to right.
 | @left@		[[type:scriptable]] [[type:double]]	''Required''	Distance between left edge of the box and the left of the card in pixels.
 | @width@		[[type:scriptable]] [[type:double]]	''Required''	Width of the box in pixels.
 | @right@		[[type:scriptable]] [[type:double]]	''Required''	Distance between right edge of the box and the ''left'' of the card in pixels.