[Bugs] #1183 HIGH: add function to detect if we are running on a XO

[Sugar Labs Bugs]

#1183: add function to detect if we are running on a XO
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    Reporter:  tomeu          |          Owner:  erikos     
        Type:  enhancement    |         Status:  new        
    Priority:  High           |      Milestone:  0.90       
   Component:  sugar-toolkit  |        Version:  Unspecified
    Severity:  Minor          |       Keywords:             
Distribution:  Unspecified    |   Status_field:  Assigned   
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Comment(by sascha_silbe):

 Replying to [comment:6 walter]:
 > From the activity developer POV, there are situations where you want to
 know if you are using the XO hardware:
 > (1) to adjust the font sizes since there is a funny scaling factor;
 In that case the scaling factor should be read and used either directly or
 more appropriately via GTK style / sugar.graphics.style. I'm setting
 SUGAR_SCALING on non-XO hardware as well and even intend to extend it to
 take values other than 72 and 100.

 > (2) know if the special sensor input modes are available;
 These should be discoverable by checking for the corresponding mixer
 controls. To be of any use the activity needs to be able to toggle them
 anyway.

 > (3) exploit the extra OLPC keyboard and panel buttons.
 I don't see a reason to make this conditional, other than maybe ignoring
 keybinding errors if the X server doesn't know the corresponding keysyms -
 i.e. the same way we handle global hotkeys.

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Ticket URL: <http://bugs.sugarlabs.org/ticket/1183#comment:7>
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