When `Settings` are shown, the 'background' actions using shortcuts should be impossible

### What
When `Settings` are open, the user can still do everything with the app using the shortcuts (e.g. starting with 'Cmd+Shift+U` to navigate to the back layer).

https://user-images.githubusercontent.com/73365754/112155455-fe9acd00-8bed-11eb-920c-f113c80f2e48.mov

### Why
Not really a bug as it is, but weird in certain cases. For example, when deleting a tag, and having no possibility to see the confirmation dialog.

### How
Either `Settings` window should be closed once it looses focus (click outside or using a shortcut that will focus other parts of the app), or making these shortcuts not work while settings are shown. Other options

cc @SylvesterWilmott

Start by reproducing the shortcut behavior with the Settings window open, using the linked recording and the Cmd+Shift+U example. Trace the Settings window focus handling and global shortcut dispatch; done means background actions cannot run while Settings is shown, or Settings closes when focus moves elsewhere, with the chosen behavior verified in the relevant interaction paths.

Written by the indexing model from the issue text.