Fix canvas size when grid display is active

 - Dynamic resizing of canvas when display_grid is activated/deactivated
 - Adding a main-wrapper to get a perfect alignement at the top and
bottom the application screen
 - fix DPI update on page load (independent of grid option).
 - fix available width for DPI calculation (using margin-box, was using
only content before)
Merge pull request #112 from juliandescottes/fix-canvas-size-with-grid

Fix canvas size when grid display is active