Add sketch mode

CREATESKETCH opens a sketch on the current UCS, or on one of the three
origin planes with XY, XZ or YZ. FINISHSKETCH closes it again.

Nothing here is new machinery: the plane is a UCS, the square-on view
is PLAN, the drafting aids are the snapper. What the pair adds is
holding them together and putting them back afterwards, including the
snap master state, so a user who works with snapping off gets that
back rather than silently keeping it.

Squaring the view up is not cosmetic. Drawing on a plane seen nearly
edge-on is the main way geometry lands somewhere the user did not
mean, because a few pixels of cursor travel cover a long way in world
space.

A sketch is a mode rather than a document object, so what it produces
is ordinary model-space geometry and the file stays a DWG other
software can open. Whatever was drawn becomes the Previous selection
set, so EXTRUDE then Previous consumes it without making the user
reselect a profile they just finished.

src/app/commands/mod.rs

@@ -739,6 +739,11 @@ inventory::submit!(crate::command::CommandRegistration {
         "SELSIM",
         // Draw a new object of the same type as the selected one. (#239)
         "ADDSELECTED",
+        // Open and close a sketch: a drawing plane, a square-on view and
+        // snapping, held together until the profile is done.
+        "BROWSER",
+        "CREATESKETCH",
+        "FINISHSKETCH",
         "SHEETSET",
         "SHORTCUTS",
         "SSM",