fix(a11y): added description for icon buttons

[mfranzke]

What does this PR do?

the icons to switch in between monthly, weekly and column view aren't providing any textual description that is e.g. relevant for screenreaders. The included self developed tooltip doesn't work as a workaround for this.

Mandatory Tasks (DO NOT REMOVE)

• [x] I have self-reviewed the code (A decent size PR without self-review might be rejected).
• [x] ~I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.~ -> N/A
• [x] ~I confirm automated tests are in place that prove my fix is effective or that my feature works.~ -> N/A

How should this be tested?

You could for example check the accessibility sub-tab after selecting the button-element within Google Developer Tools to check for the included description, see the following screenshot with the current result and missing description:

Checklist

• I haven't checked if my changes generate no new warnings