Improve the shortcut behavior

Problem/Motivation

There are currently three known problems:

1. Due to https://github.com/skipto-landmarks-headings/page-script-5/issues/13 if you have an input field in focus and press option 0, instead of invoking the SkipTo menu the ≠ character is shown.
2. If you are in a text area aka a ckeditor 5 text field instead of invoking the SkipTo menu, the Accessibility help menu of ckeditor 5 is shown instead.
3. if you have a dialog modal open the SkipTo menu is still accessible. With the popup option selected for SkipTo you are able to display the SkipTo menu, but you are unable to navigate the available option by keyboard, and if you press the escape button the dialog modal closes and the skipto menu remains expanded. but if you select one of the options instead by using the mouse the picked option is visually highlighted in the background of the dialog modal. with the fixed and static options the button is shown in the foreground alongside the dialog modal and is clickable by the mouse cursor, but unreachable by keyboard.

Steps to reproduce

for 1: go to a node edit form, get a single line input field in focus, and press option 0
for 2: go to a node edit form, get a multi line wsiwyg body field in focus and press option 0
for 3: go to a view, and click one of the field links to invoke a dialog modal and then test with either fixed, static and or the popup option active.

Proposed resolution

for 1 and 2: it will open up an issue upstream for suggesting reintroducing an option on the addon to adjust the shortcut. that way it might be changed from option 0 to ctrl 0 or something else?

for 3: not sure if that is also necessary to solve upstream or if it could be solved within the skipto issue queue on d.o, but i would suggest to disable skipto in case a dialog modal is active. so in case popup/popup border is selected no skipto menu is being invoked and in case fixed/static is selected the button is removed/hidden or at least the buttons z-index is being reduced so the button becomes unreachable in the background of the dialog modal.

Remaining tasks

User interface changes

API changes

Data model changes