Allow to switch to all empty buffers (read-only as well)

Allow to switch to all empty buffers (read-only as well)

* ace-window.el (aw-window-list): Stop filtering out empty read-only
  buffers.
(aw-empty-buffers-list): New defvar, store empty buffers here.
(aw--done): Return empty buffers to their empty state (they had a single
space inserted in order for overlay to work).
(aw--lead-overlay): Insert a space into an empty buffer.
(aw-select): Move inserting a space into `aw--lead-overlay'.