improved emoji support (unicode TR51)

Submitted by Christian Persch @chpe

Link to original bug (#767529)

Description

In unicode 9, there's a change to East_Asian_Width property that makes emojis Wide, see http://www.unicode.org/reports/tr11/tr11-30.html :

''' ED4. East Asian Wide (W): All other characters that are always wide. These characters occur only in the context of East Asian typography where they are wide characters (such as the Unified Han Ideographs or Squared Katakana Symbols). This category includes characters that have explicit halfwidth counterparts, along with characters that have the [UTR51] property Emoji_Presentation, with the exception of the range U+1F1E6 REGIONAL INDICATOR SYMBOL LETTER A through U+1F1FF REGIONAL INDICATOR SYMBOL LETTER Z. '''

and

''' Emoji style standardized variation sequences behave as though they were East Asian Wide, regardless of their assigned East_Asian_Width property value. '''

So it's not as simple as just change coming through glib's iswide[_cjk] API, and we'll need to have access to the Emoji_Presentation property (either through glib or simpler by adding it to vte itself).