Filter binding label

[VeeDeltaVee]

Update the controller (filter_controller.go) to set up the binding to be displayed. Additionally, update the translations for the binding name to be shorter, to make it more likely the text will fit at the bottom of the screen

The keybinding screen in particular (which is a filterable popup) seems frustrating for users. In #3141, @kadaan suggested that perhaps the keybindings page should display a message in the footer about the presence of this option as a way to increase discoverability.

I ran into this issue myself, and figured I'd see if anyone else had implemented some kind of fuzzy finding for the keybindings menu (somewhat like what aerc has). The issue I linked earlier seemed to describe the perfect solution, and it was easy enough to implement. Perhaps in the future the keybinding popup can have special functionality where typing automatically filters, but just having the filter option shown at the bottom seems to me a big improvement.

For reference, here's what the keybindings menu looks like with the change (notice the new "Filter" keybinding hint at the bottom):

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[VeeDeltaVee]

I've marked the adding tests task as unfinished, although I'm not sure if this would even need tests. If it does, please let me know and I'll add some.