Improve layout and styling of Go to File window.

- Remove focus ring from search field as there is no second target.
- Set top and bottom border widths for the textured window.
- Remove miniaturize and zoom buttons for lack of utility.
- Match name of window to menu command.
- Various color and spacing tweaks.
Use ns::create_event_string
Fix unreliable tab switching keys

This was broken by 645f8d91e14cfc9b9b45aab3701378949e142e65.

The issue is that if the user press a key not in our managed menu, then the menu delegate will no longer be asked to update the menu for keys not in this (cached) menu.

Example: If there are items for ⌘1-3 in the menu and the user press ⌥⌘N a few times (to create more tabs), then the delegate is asked to update the menu on the first press, but as ⌥⌘N is not in the menu, successive events will not cause the menu to be updated. If the user then press ⌘5 (to go to the 5’th tab), nothing will happen, because our menu only holds 3 items, and the delegate is not asked to update the menu.
Don’t open documents with a path via UUID

The UUID is a unique reference to a document that trumps the path. However, as we don’t actually keep the documents in the file chooser, we end up with UUIDs from documents that does not exist, and should a new document be created for the same path, it will get another UUID.
Improve scroll when selecting units

When selecting a unit (Edit → Select submenu or e.g. Find Next) then we only scroll if we can make the entire selection visible.

Closes #692.
Files with no extension open as plain text

This is instead of asking the user.