tiny-mce-anchor isn't compatible with tinymce5

[Teetrinker]

It seems like this anchor-plugin is not compatible with Contao 4.13 because Contao4.10 and upwards use tinymce5.

**Tries & errors **
I updated a Contao installation from 4.9 to 4.13 and couldn't get the anchor plugin to work.

First I got this javascript error:

anchor Error: editor.addButton has been removed in tinymce 5x, use editor.ui.registry.addButton or editor.ui.registry.addToggleButton or editor.ui.registry.addSplitButton instead

Trying to fix this by hand led to more issues:

Could not find valid required value for "onAction"

[cliffparnitzky]

Thank you for the hint and sorry for the inconvenience.

Because I will not continue to maintain the extension, I won#t try to fix the issue.

Please switch over to modify the template be_tinyMCE.html5.