Clicking the toolbar button twice or triggering the context menu item
twice or activating the shortcut twice is highly inconvenient for those
who prefer to keep the auto-sync option off. Toggling auto-sync should
be less frequent and thus doesn't require a shortcut, can be hidden in a
tool button's popup menu.

Two actions with the same function (syncActionMenu and syncSubAction)
are necessary to allow both locating the current document and toggling
auto-sync from the context menu when Projects Toolbar is hidden.

Enable/disable all 3 actions in updateSyncAction(). Enabling/disabling
only syncActionMenu does not prevent calls to
ProjectManagerView::locateCurrentDocument() via existing and possible
future shortcuts of the two sub-actions.