provide feedback during provenance graph syncing

[sgratzl]

at the moment there is no indication that the graph is being synced with the server side. In addition, the user should be warned if she closes the window while syncing is still active.

[sgratzl]

e.g. in Ordino the icon is pulsing (https://github.com/Caleydo/ordino/blob/develop/src/EditProvenanceGraphMenu.ts#L43)