Add the Browser panel

A docked outline of the drawing: the three origin planes, the open
sketch, and every solid labelled with the operation that made it,
recovered through primitive_property_operation. Rows run ordinary
commands, so clicking a plane starts a sketch on it and clicking a
body selects it.

It is a table of contents, not a feature tree. The DWG history records
how each solid was built but nothing re-runs it, so an earlier feature
cannot be edited to rebuild the ones after it. Presenting the list as
editable history would promise exactly that, so the panel states what
it is instead.

Off by default and opened with BROWSER, leaving the existing dock
layout alone for anyone who does not want it.

src/ui/window/mod.rs

@@ -1,5 +1,6 @@
 pub mod about;
 pub mod block_palette;
+pub mod browser;
 pub mod layout_manager;
 pub mod layer_state_manager;
 pub mod drawing_units;