[Bugs] #1317 UNSP: check for Rainbow binaries before trying to use them

[SugarLabs Bugs]

#1317: check for Rainbow binaries before trying to use them
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    Reporter:  sascha_silbe               |          Owner:  sascha_silbe     
        Type:  defect                     |         Status:  accepted         
    Priority:  Unspecified by Maintainer  |      Milestone:  0.86             
   Component:  sugar-toolkit              |        Version:  Git as of bugdate
    Severity:  Unspecified                |     Resolution:                   
    Keywords:  r?                         |   Distribution:  Unspecified      
Status_field:  Assigned                   |  
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Comment(by sascha_silbe):

 _rainbow_enabled is an instance (not class) attribute because it executes
 a binary (didn't want to add a function to replicate exec*p() behaviour),
 so is quite expensive. Storing a boolean is quite cheap OTOH.

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Ticket URL: <http://dev.sugarlabs.org/ticket/1317#comment:4>
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