Closes #6394

That option was apparently left over from a debugging session

Closes #6408

Which kept the cursor blink and input poll intervals alive, leaking
memory.

Closes #6410

Issue #6412

If we are given the following snippet of text:

```
A green green sky.
  _
```

We can search for the word "green" with `/green`, and then use `gn` to
select the next occurrence of "green" in visual mode.

```
A green green sky.
  -----
```

Alternatively, we can use `cgn` and then enter the word "blue" to change the
word "green" to "blue".

```
A blue green sky.
```

Then we can use the `.` operator to repeat the change:

```
A blue blue sky.
```

Addresses #3851.

Closes #6414

This changes lint.css to be less reliant on the predefined severities
(error and warning), in turn making it easier to define custom ones.
Now all that needs to be done in order to define a new severity, e.g.
`note`, is to add the following CSS:

```css
/* underline */
.CodeMirror-lint-mark-note {
  background-image: ...;
}

/* icon */
.CodeMirror-lint-marker-note, .CodeMirror-lint-message-note {
  background-image: ...;
}
```

Previously, it was necessary to copy many styles that were only
available under the `CodeMirror-lint-*-error` and
`CodeMirror-lint-*-warning` classes.

By swapping the CSS rules, the error rules take priority in case there
are markers with both severities on the same token. That token is now
underlined red instead of yellow, making it consistent with how errors
take priority in the gutter.

…read-only

This prevents cut/paste from showing up in the context menu on Chrome
(but doesn't help on Firefox).

Closes #6418

Closes #6420:

Issue #6422