New command m-x eglot-code-actions

New command m-x eglot-code-actions

Also available when left-clicking diagnostics.

* README.md: Mention eglot-code-actions. Slightly rewrite
differences to lsp-mode.

* eglot.el (eglot-code-actions): New command.
(eglot-handle-notification :textDocument/publishDiagnostics): Use
eglot--make-diag and eglot--overlay-diag-props.
(eglot--mode-line-props): Use eglot--mouse-call.
(eglot--mouse-call): Renamed from eglot--mode-line-call.
(eglot-client-capabilities): List :executeCommand and :codeAction
as capabilities.
(eglot--diag, advice-add flymake--highlight-line): Horrible hack.
(eglot--overlay-diag-props): Horrible hack.