Tree editor extension custom menu entry should handle no open workspace

[lucas-koehler]

At the moment, the tree editor extension's Tree Editor -> New Example File menu entry does not work when there is no workspace open in Theia.
In this case, the menu Tree Editor is empty and an error root ERROR Child node 'open-editors:root' does not belong to this 'open-editors:root' tree. is logged. This is confusing for users.
Instead, the menu entry should be visible but disabled and ideally have a tooltip describing that a workspace needs to be open to use it.