Fix dropdown menu visibility when used inside KModal

[KshitijThareja]

Description

This PR fixes the dropdown menu's visibility when we use KDropdownMenu in a KModal.

Issue addressed

Addresses #486

Before/after screenshots

Before:

After:

Changelog

• [Fix dropdown menu visibility when used inside KModal #493]
• Description: Changed the value of z-index of KDropdownMenu to 99.
• Products impact: Bugfix
• Addresses: KDropdownMenu in KModal: Dropdown menu does not display in KModal #486
• Components: KDropdownMenu
• Breaking: No
• Impacts a11y: No
• Guidance:

Steps to test

1. Add the code given in the linked issue to the file mentioned there.
2. Navigate to KModal documentation page to test usage of KButton and KDropdownMenu

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical and brittle code paths are covered by unit tests
• The change is described in the changelog section above

Reviewer guidance

• Is the code clean and well-commented?
• Are there tests for this change?
• Are all UI components LTR and RTL compliant (if applicable)?
• Add other things to check for here

After review

• The changelog item has been pasted to the CHANGELOG.md

[MisRob]

Thanks, @KshitijThareja, we will review.

[LianaHarris360]

@KshitijThareja Thanks for working on this! The code change fixes the problem and I haven't noticed any side effects within Kolibri. Before we can approve and merge this PR, the CHANGELOG.md file will need to be updated to include the info about this change.

[KshitijThareja]

@LianaHarris360 I've updated the CHANGELOG.md with the info about this particular change. I guess it can be merged now.
Thanks :)

[MisRob]

Great @KshitijThareja, thanks!