[Feature] Dynamic columns

Calendar component:

• active/non-active columns
• active columns are displayed
• non-active columns are hidden

Tree classes and interfaces

HierarchicalColumnFilter component:

• props.flatNodes: id and link to parent id
• v-model:activeIds: list of ids
• use the Tree classes
• display a tree on a grid with flex: grid
• control each cell content via slots

To be decided/changed/added:

• I guess we should remove the hidden events from the DOM, not only hide them with null width
• link HierarchicalColumnFilter to the columns in Calendar