Allow fast publish and cancel actions when previewing posts.

Addresses #1285 by adding a key handler to the preview sheet.
It also put the focus on the cancel button. The result is that after
entering preview with `<Ctrl/Cmd> + <Return>` ...

* `<Return>` OR `<Escape>` will cancel the publish and go back to editing
* `<Ctrl/Cmd> + <Return>` once more will publish the message.

Author: black-puppydog

lib/depject/message/sheet/preview.js

@@ -46,6 +46,7 @@ exports.create = function (api) {
       // allow inspecting of raw message that is about to be sent
       messageElement.msg = msg
 
+      const cancelButton = h('button -cancel', { 'ev-click': cancel }, i18n('Cancel'))
       return {
         content: [
           messageElement
@@ -73,8 +74,22 @@ exports.create = function (api) {
             ])
           ),
           h('button -save', { 'ev-click': publish }, i18n('Confirm')),
-          h('button -cancel', { 'ev-click': cancel }, i18n('Cancel'))
-        ]
+          cancelButton
+        ],
+        onMount: () => {
+          cancelButton.focus()
+        },
+        attributes: {
+          'ev-keydown': ev => {
+            if (ev.key === 'Enter' && (ev.ctrlKey || ev.metaKey)) {
+              publish()
+              ev.preventDefault()
+            } else if (ev.key === 'Escape') {
+              cancel()
+              ev.preventDefault()
+            }
+          }
+        }
       }
 
       function publish () {