Adjust template of widgets

When adding own widgets, it might be necessary to provide custom templates. In such a case the file path containing the template files needs to be added.

This is done using a Configuration/page.tsconfig file, see changelog and changelog for details on this:

# Pattern: templates.typo3/cms-dashboard."something-unique" = "overriding-extension-composer-name":"entry-path"
templates.typo3/cms-dashboard.1644485473 = myvendor/myext:Resources/Private

A template file can then be added to path Resources/Private/Templates/Widgets/MyExtensionsGreatWidget.html and is referenced in the PHP class using ->render('Widgets/MyExtensionsGreatWidget');. The registration into namespace typo3/cms-dashboard is shared between all extensions. It is thus a good idea to give template file names unique names (for instance by prefixing them with the extension name), to avoid situations where templates from multiple extensions that provide different widgets override each other.