T20 round 1 fixes

- the hex demo's selection stops at memory's end
- a disassembly gutter fills its row, so an empty one takes a press
- a breakpoint's box keeps its press from the row
- ruler ticks keep the places their step needs
- the toolbar names each usual key for the platform; the app binds them
- tests: gutter press, box press, tick places

src/debug/mod.rs

@@ -8,6 +8,8 @@ mod hex;
 mod network;
 mod session;
 mod stack;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
 mod timeline;
 mod variables;