Context menus stay in view near the window edge; planes offer Create Sketch (#1398)

List menus open below-right of the cursor and flip to end at the cursor where they don't fit (shared context_menu::place / show for viewport, browser, toolbar and Select Other). Right-clicking a plane in the viewport, a construction plane row or an origin plane row (new menu with Create Sketch and Show/Hide) offers Create Sketch on that plane. Fixes #1354.

docs/book/src/guide/ui-tour.md

@@ -45,7 +45,8 @@
   home, framing what is shown now (Navigation › **Middle double-click** can make it Fit, which
   keeps the orientation, or turn it off).
   Right-click for the marking menu: eight commands around the cursor, plus a
-  menu for whatever is under it.
+  menu for whatever is under it. Near the window's bottom or right edge a menu opens above or
+  to the left of the cursor, so it stays in view.
   Shaded views omit certified smooth joins between spline spans on extruded
   walls. Real creases, cap outlines and authored face splits remain visible.
   Wireframe retains its edge presentation, and hiding a join does not remove its CAD