[Bugs] #4398 sugar-toolkit-gtk3 UNSP: Toolkit: add MessageBox

[Sugar Labs Bugs]

#4398: Toolkit: add MessageBox
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    Reporter:  manuq                      |          Owner:  manuq      
        Type:  enhancement                |         Status:  new        
    Priority:  Unspecified by Maintainer  |      Milestone:  1.0        
   Component:  sugar-toolkit-gtk3         |        Version:  Unspecified
    Severity:  Unspecified                |       Keywords:  design     
Distribution:  Unspecified                |   Status_field:  Unconfirmed
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 In the shell and activities we are adding informative messages more and
 more.  The look is: a centered message in the canvas, with an icon, a bold
 title, optional description, and optional buttons.  This elements are
 horizontally centered aligned too.  Example:

 http://bugs.sugarlabs.org/attachment/ticket/4384/downloading.png

 We should add a new widget MessageBox to remove the duplicated code,
 improve consistency, and future usage.

 Replacements:

  * Journal "Is empty"
  * Journal "No matching entries", button "Clear search"
  * Home ListView "No matching entries", button "Clear search"
  * Browse "Downloading document...", button "Cancel"
  * Read "No book", button "Add book"
  * Jukebox "No media", button "Add media"

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Ticket URL: <http://bugs.sugarlabs.org/ticket/4398>
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