Capture notice in a popout shows [object DocumentFragment] and no Undo button #498

When a capture runs while a popout window has focus, the timestamp is saved correctly, but the notice reads [object DocumentFragment] and has no Undo button.

src/ui/undoNotice.ts builds the notice from a fragment created with activeDocument.createDocumentFragment(), which is the popout's document. Obsidian's Notice appears to test for a fragment from the main window, so the popout fragment is turned into a string.

Seen on Obsidian 1.14.4 (Linux) on master before and after #494, during the verification of #494. Evidence: master and head popout notice snapshots and a screenshot of the malformed notice.

Expected: the notice shows its message and a working Undo button, whichever window has focus.