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/sketching.md

@@ -1,8 +1,9 @@
 # Sketching and constraints
 
 A sketch lies on a plane: an origin plane, a construction plane or a planar face. Start one with
-**Create Sketch** (`sketch.create`) and click a plane, or pick a face first. **Finish Sketch**
-returns to the model.
+**Create Sketch** (`sketch.create`) and click a plane, or pick a face first. Right-clicking a
+plane, in the view or on its browser row, offers **Create Sketch** on that plane too.
+**Finish Sketch** returns to the model.
 
 In scripts, `XY`, `XZ`, `YZ` and their `Top`, `Front`, `Right` aliases name origin planes.
 These names are reserved: a construction plane cannot be created with or renamed to one