[Pidgin] #14649: Username validation

[Pidgin]

#14649: Username validation
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 Reporter:  tomkiewicz   |     Owner:        
     Type:  enhancement  |    Status:  new   
Component:  libpurple    |   Version:  2.10.0
 Keywords:  api          |  
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 We could introduce (in 3.0.0 API) new method "validate" in
 !PurplePluginProtocolInfo, similiar to normalize. It would check, if
 provided username is usable at all: for example, Gadu-Gadu uses numbers as
 usernames, so anything containing letters can be identified as invalid
 username.

 This method could be used in various places:
  * account add/create dialog
  * buddy add dialog
  * new message dialog
 In these places, Pidgin now only checks, if provided username is not
 empty.

 It could just return boolean, or string:
  * the same string as in parameter, if username is valid
  * somehow altered string, if username is invalid, but easy to fix (for
 example trim whitespaces)
  * NULL if username is invalid

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Ticket URL: <http://developer.pidgin.im/ticket/14649>
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