Update special arabic support to UCS4

Update special arabic support to UCS4

	* src/encoding.C
	(arabic_table): Delete (this was used for fonts in iso8859-6 encoding)
	(arabic_table2): Rename to arabic_table and update code points to UCS4.
	The old code points in this table where a custom encoding:
	Some ASCII positions and some unused positions of the iso8859-6
	encoding where used to encode the shaped forms. Example:
	The character hamza was encoded with 0x41. The iso8859-6 encoding in
	lib/encodings contained ithe value 0xfe80 at position 0x41 before
	the transition to unicode => Change the value 0x41 to 0xfe80 in the
	table.
	This old custom encoding was needed because the transformed characters
	needed to be transported through 8bit characters.
	(Encodings::is_arabic_special): Change code points from iso8859-6 to UCS4
	(Encodings::isComposeChar_arabic): Ditto
	(Encodings::is_arabic): Ditto
	(Encodings::transformChar): Always use arabic_table

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17532 a592a061-630c-0410-9148-cb99ea01b6c8