fix bug on multiple creation of alternative tasks + snackbar alert on transform and delete alternative tasks

Changes to the tutorial + Removal of conditions on prerequisites (adding a condition on tasks other than subtasks preventing subtask prerequisites) + Addition of a condition for a unique common task symbol in a role + Fix clean-dialog text

margin in mat-menu / new tooltips / replace purger by effacer / fix add role dialog message  / zoomin and zoomout with buttons / click on a piece show details + remove icon button to show detail / set font-family everywhere

add comments sider-piece / resize repeat sider-piece textarea / add mat-button / replace symbolchoix and prerequires buttons by mat-buttons