Closes #ISSUE

Previously, the keystroke input would be empty, even when editing an
existing binding. This meant you had to re-enter the bindings even if
you just wanted to edit the context. Now, the existing keystrokes are
rendered as a placeholder, are re-shown if newly entered keystrokes are
cleared, and will be returned from the `KeystrokeInput::keystrokes()`
method if no new keystrokes were entered.

Additionally fixed a bug in `KeymapFile::update_keybinding` where
semantically identical contexts would be treated as unequal due to
formatting differences.

Release Notes:

- N/A *or* Added/Fixed/Improved ...