Add desktop-registry-remove-desktop

- desktop-registry.el (desktop-registry--completing-read):
  (desktop-registry-remove-desktop): New functions.
  (desktop-registry-change-desktop): Use
  `desktop-registry--completing-read' to interactively ask the user
  which desktop to switch to.
Add desktop-registry-current-desktop

* desktop-registry.el (desktop-registry--canonicalize-dir):
  (desktop-registry-current-desktop): New functions.
  (desktop-registry-add-directory): Use
  `desktop-registry--canonicalize-dir' to get the "clean" directory
  name.
Add desktop-registry-rename-desktop

* desktop-registry.el (desktop-registry-rename-desktop): New function.
Change prompts when selecting a desktop
Enable use of current desktop as default

When asking the user for a desktop it is now easy to specify that the
default should be the current desktop.

- desktop-registry.el (desktop-registry-current-desktop): Add optional
  DEFAULT parameter which specifies the return value when
  `desktop-dirname' is `nil', instead of previously an empty string.

  (desktop-registry--completing-read): Add optional DEFAULT-CURRENT
  parameter which specifies that the current desktop should be the
  default when asking for a desktop.

  (desktop-registry-remove-desktop): (desktop-registry-rename-desktop):
  Use the current desktop as default when asking the user to select a
  desktop.
Update docstrings
Move lexical binding specification

This is to keep (one of) the Emacs Lisp syntax checkers happy.
Signal error and specify name

- desktop-registry.el (desktop-registry-add-directory): Add an optional
  NAME parameter which will be asked for (in an interactive context)
  when the universal argument is used. Print a message if the specified
  directory has already been registered, signal an error when the (given
  or generated) name has already been used and only add the directory if
  neither of these conditions occur.

  (desktop-registry-add-current-desktop): Add optional NAME parameter
  which will be asked for (in an interactive context) when the universal
  argument is used and pass it on to `desktop-registry-add-directory'.
Remove title from README

It isn't used by anything and doesn't render well in the github output.
Bump version to 1.1.0