(core) fixes hanging Save/Revert tooltips

Summary:
 - Clicking quickly on the small save/revert button was caussing the
 tooltip to stay around.
    - But if user waited a little bit before clicking the save button,
 the tooltip was shown, and then properly removed when the button was removed.

 - Code was missing propertly handling of disposal before the tooltip
   were shown.

Test Plan: Added test case to the projects/tooltip.ts tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2797

app/client/ui/tooltips.ts

@@ -174,6 +174,9 @@ export function setHoverTooltip(refElem: Element, tipContent: ITooltipContentFun
     // If request, re-open on click.
     dom.onElem(refElem, 'click', () => { close(); open(); });
   }
+
+  // close tooltip if refElem is disposed
+  dom.onDisposeElem(refElem, close);
 }
 
 /**