[editor] Rename selectedSectionIndex

...ts/editor/src/app/components/canvas/canvas.component.html

...ects/editor/src/app/components/canvas/canvas.component.ts

...ditor/src/app/components/canvas/section-menu.component.ts

...perties-tab/input-groups/dimension-field-set.component.ts

...operties-tab/input-groups/position-field-set.component.ts

projects/editor/src/app/services/selection.service.ts