devel/okteta: update to 0.26.8

Changes since 0.26.7:

• Fixed: Float 32-bit editor in decoding table tool no longer rejects negative values & decimal places
• Fixed: PNG structure definition example now ensures use of big endianness
• Improved: translations
• Improved: have more text vertically centered in lists in tool views
• Changed: no more use of fixed font type for any list headers in tool views