[BITV]: Rework native tooltips on NcAvatar

[JuliaKirschenheuter]

Problem

To show a tooltip on a several components we currently use v-tooltip. An example can be found in https://github.com/nextcloud/nextcloud-vue/blob/master/src/components/NcAvatar/NcAvatar.vue#L112

v-tooltip is not accessible and we want to migrate to native tooltips (title attribute) instead. An overview can be found in #2503.

Solution / ToDo:

• Add a title prop to NcAvatar component that will render as title attribute on the HTML element. Remove custom tooltips.
• Show valid value for aria-label every time.