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/buttons/mod.rs

@@ -15,6 +15,7 @@ mod sheet;
 mod tests;
 
 pub use bars::{ActionBar, BulkActionBar};
+pub(crate) use button::shortcut_text;
 pub use button::{Button, ButtonVariant};
 pub use confirm::{ConfirmButton, ConfirmMode};
 pub use copy::CopyButton;