Avoid adding unnecessary <style> elements.

We used to add the same one over and over again.
Gateway: Added a delete button.

Each file in the gateway can display a menu in which a functional delete
button is.
Include the file menu while searching.
File Menu on the toolbar.

Includes the Delete button for now.
gateway: Close the file menu when selecting a file.
Gateway: gradient b/w toolbar and file menu.
Gateway: toggle menu internally upon file selection.
Delete button menu. Works for multiple files.

Also works while searching files.
Gateway: Ask for confirmation upon deletion.
Gateway: Correct plural when deleting files.
gateway: Delete file(s) (take plural into account).
gateway: keyboard selection [jkx].
gateway: reset event listeners after file deletion.
Gateway: deletefilebut → deletefiles

As a suggestion from Jan Keromnes.