Place: switching, opening or closing a document leaves the place cursor's tool

Review fix for #2309: set_active, add_document and close_document emptied the place cursor but
kept the placeGun tool, so the next canvas click failed with 'nothing loaded in the place
cursor'. One helper, Session::unload_place_cursor, empties the cursor and its queue and goes back
to the Selection tool when the place cursor's tool is active; validate_place_cursor uses it too.
Test: changing_documents_unloads_the_place_cursor_and_its_tool.

docs/multi-image-placement.md

@@ -7,6 +7,7 @@ Choose **File > Place** (Cmd+D on macOS) and select several images. They load in
 - Click an existing graphic frame to replace its contents.
 - While drawing, Right/Left add/remove columns and Up/Down add/remove rows. Release to place a grid in row order with 6-point gutters. Extra images remain in the cursor; unused grid cells create no empty frames. A drag too small for the grid's cells shows and places one frame instead, as an ordinary drag does.
 - Press Escape or select another tool to cancel the remaining placement.
+- Switching to, opening or closing a document also cancels it, and goes back to the Selection tool.
 
 A single selected file retains the normal Place behavior, including filling a selected frame and PDF import options. Image batches support up to 1000 images. Rows and columns each range from 1 to 100. A batch containing an unreadable image fails without replacing the existing queue or changing the document. An invalid grid area also leaves the document and queue unchanged.