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)
Exclude add-frame-button from sortable items in preview-film
Changed sortable items selector to a more robust one
Merge pull request #113 from juliandescottes/fix-addframe-button-sortable

Exclude add-frame-button from sortable items in preview-film
Merge pull request #112 from juliandescottes/fix-canvas-size-with-grid

Fix canvas size when grid display is active
Merge branch 'master' into gh-pages