Fix ‘QTags is not defined’ error

[pewgeuges]

JCV fixes the QTags button after WP 5.8 failed it again:

https://wordpress.org/support/topic/qtags-addbutton-take-two/

As mentioned in the closing topic https://wordpress.org/support/topic/qtags-addbutton/ and researched in Footnotes’ version history, a fluctuating QTags support by WordPress affected some versions:

• WP 4.8: QTags fails, @ericakfranz provides fix;
• WP 5.5: QTags works;
• WP 5.6: QTags fails, DPartridge reports bug;
• WP 5.7: QTags works;
• WP 5.8: QTags fails, JCV reports bug and fixes it.

[Rumperuu]

wontfix for 2.7.4 as the issue is non-urgent and the codebase to edit will be changing anyway; will be included in 3.0 instead

Related: #15

See also https://developer.wordpress.org/apis/handbook/quicktags/#examples