Implemented Sprite Manager. Add and delete sprites syncronized with the sprite data model and the canvas.

Styled the Sprite Manager.
Synchronized the sprite drop down lists in the command views to the sprite table.

Added the sprite drop down lists to all the command templates that requires it.
Added icons to manager views.
Changed icons in command views.
Merge branch 'master' of github.com:chewonit/cs3213-visual-ide-team20
Binded the variable number toggle button click event to the document instead.