Added undo-history warning when opening file using File API, fixing rest of issue 459
Added undo-history warning when opening file using File API, fixing rest of issue 459

ryanramage/svg-editor,ryanramage/svg-editor,ryanramage/svg-editor,trentmillar/ddp-level-editor,trentmillar/ddp-level-editor,trentmillar/ddp-level-editor