Merged PR 162: Implement symbols and other stuff (see desc)

Implement symbols
- Add, Remove, Select Container
- Form
- Link with container
- Symbol behavior application to container (move to x with xpositionreference)

Important changes
- Remove SelectedContainer from HistoryState, meaning that it will be slower for each load but will be faster for each operations* (SetHistory, SelectContainer, DeleteContainer, SymbolOperations)
- ElementsSidebar now opens with isSidebarOpen meaning that both sidebar will open on toggle
- Moved camelize, transformX, restoreX to different modules (stringtools.ts, svg.ts)

src/Components/Editor/Behaviors/AnchorBehaviors.ts

@@ -21,7 +21,7 @@ import { constraintBodyInsideUnallocatedWidth } from './RigidBodyBehaviors';
  * Apply the following modification to the overlapping rigid body container :
  * @param container Container to impose its position
  */
-export function ImposePosition(container: IContainerModel): IContainerModel {
+export function ApplyAnchor(container: IContainerModel): IContainerModel {
   if (container.parent === undefined ||
     container.parent === null) {
     return container;