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/Interfaces/ISymbolModel.ts

@@ -0,0 +1,24 @@
+import { IAvailableSymbol } from './IAvailableSymbol';
+
+export interface ISymbolModel {
+  /** Identifier */
+  id: string
+
+  /** Type */
+  type: string
+
+  /** Configuration of the symbol */
+  config: IAvailableSymbol
+
+  /** Horizontal offset */
+  x: number
+
+  /** Width */
+  width: number
+
+  /** Height */
+  height: number
+
+  /** List of linked container id */
+  linkedContainers: Set<string>
+}