First stab at a Flymake diagnostics buffer

First stab at a Flymake diagnostics buffer

The diagnostics buffer summarizes the diagnostics of a buffer in a
tabulated list and is permanently updated after each Flymake check.

* lisp/progmodes/flymake.el (flymake--handle-report): Call
flymake-show-diagnostics-buffer under certain conditions.
(flymake-menu, flymake--diagnostics-buffer-source)
(flymake--diagnostics-buffer-button-keymap)
(flymake-show-diagnostic-at-point)
(flymake-goto-diagnostic-at-point)
(flymake--diagnostics-buffer-entries)
(flymake-diagnostics-buffer-mode)
(flymake--diagnostics-buffer-name)
(flymake-show-diagnostics-buffer): New definitions.