Introduce TemplateTrait to fix missing method in Widget

[fritzmg]

This PR fixes the following things:

• Added the missing template methods to Contao\Widget (fixes CSP breaks the form fields #6722) by introducing a new TemplateTrait that contains all public member methods that are independent from the Template abstract class.
• Fixing that also revealed another issue: two of our (front end) templates use hard coded style attributes by default, which will cause a CSP violation. This is fixed by adding hashes for these inline styles.

Note: for CSP Level 3 capable browsers this also requires the unsafe-hashes source to be present in the directive, otherwise CSP hashes do not work for inline style attributes.

Note: many of our PHP front end templates also still have

<?php if ($this->style): ?> style="<?= $this->style ?>"<?php endif; ?>

in them, but I deliberately disregarded those, as these are mostly there only for BC.

[leofeyer]

Thank you @fritzmg.