main navigation | accessibility Improvements

[sascha-mueller]

• test titles with screenreader
• Sub-navigation should close on ESC or when leaving the sub-navigation
• add vibillity hidden if navigation is hidden
• fix icons in high-contrast mode

[zoglo]

should have been fixed in #59:

• Sub-navigation should close on ESC or when leaving the sub-navigation
• fix icons in high-contrast mode

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those still remain:

• test titles with screenreader
• add vibillity hidden if navigation is hidden

[zoglo]

We may want to replace the navigation with the mod_navigation_accessible template and adjust the navigation in the core to be more flexible (be it with more blocks?) 🤔.