Make modals accessible for screen readers

[klues]

Accessibility of modals for screen readers currently is very limited. The modal for "set navigation" in edit mode has better accessibility, which also should be added to other modals, at least:

• set correct properties aria-modal="true" and role="dialog" , see setNavigationModal.vue:5
• set focus within the modal, using v-focus or .focus(), either on an input element or on the "X" close link at the top, see setNavigationModal.vue:16

This change shouldn't be done on each modal manually, but a new Vue component like baseModal.vue should be created which takes care of all basic things for modals and is used by all other modals.

[klues]

Since there is now the new HTML5 element dialog, this should be used instead of the aria and role properties, see: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dialog