add-editor-keymap-functions in Editors

Given a keymap% object, the keymap is loaded with mappable functions that apply to all editor<%> objects:

• ``copy-clipboard''

• ``copy-append-clipboard''

• ``cut-clipboard''

• ``cut-append-clipboard''

• ``paste-clipboard''

• ``paste-x-selection''

• ``delete-selection''

• ``clear-selection''

• ``undo''

• ``redo''

• ``select-all''

• (add-editor-keymap-functions keymap) -> void
     keymap : keymap% object