It is possible to change the window at 'Enter new shortcut' - this seems like a freeze

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
When you want to change a keyboard shortcut, it is possible to go back with the mouse button…then switchboard is at the startpage(?) and it seems like the system is frozen, but switchboard-plug-keyboard still waits for 'Enter new shortcut'…then the only option is [`ESC`]

### To Reproduce
Steps to reproduce the behavior:
1. Go to 'switchboard-plug-keyboard'
2. Click on 'shortcuts' and then Click any setting…take the first for example 'Close windows'…
3. Click the back button on your mouse > window change from keyboarad-plug to switchboard
4. Now nothing is possible…mouse cursor is functional but no mouse button anymore.
Seems like a crash or freeze. 🤯️
5. Press ESC and you are back again 😬️

### Expected behavior
At this point, when you must enter the new shortcut for a specified action, switchboard-plug-keybord should not be aalowed to change its window state with the mouse-back-button.


Please check what applies:
- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version

## Additional context
Maybe this is a related bug… #122

Start by reproducing the issue in switchboard-plug-keyboard: open Shortcuts, choose Close windows, begin entering a shortcut, then use the mouse back button. Trace the shortcut-capture state and window navigation; done means navigation is blocked during capture and ESC still returns normally.

Written by the indexing model from the issue text.