[Bugs] #1089 UNSP: sugar.logger: add trace decorator

[SugarLabs Bugs]

#1089: sugar.logger: add trace decorator
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    Reporter:  sascha_silbe               |          Owner:  tomeu            
        Type:  enhancement                |         Status:  new              
    Priority:  Unspecified by Maintainer  |      Milestone:  0.86             
   Component:  sugar                      |        Version:  Git as of bugdate
    Severity:  Unspecified                |       Keywords:                   
Distribution:  Unspecified                |   Status_field:  New              
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 The attached patch adds a trace decorator to sugar.logger. It can be used
 to trace function calls and limits the size of the items it logs. This way
 e.g. the "preview" attribute (metadata) will be shortened automatically.
 It will only trace calls if the new log level TRACE is in effect, which
 can be done by setting SUGAR_LOGGER_LEVEL to one of the new values 'trace'
 (lower than 'debug', i.e. 'debug' will be enabled as well) or 'all' (log
 everything).

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Ticket URL: <http://dev.sugarlabs.org/ticket/1089>
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