feat: APP-2761 - Introduce z-index component customisations for the Dropdown and Dialogs components

[cgero-eth]

Description

• Introduce component customisations for the z-index property of the Dropdown and Dialogs components

Task: APP-2761

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran all tests with success and extended them if necessary.
• I have updated the CHANGELOG.md file in the root folder of the package after the [UPCOMING] title and before
  the latest version.
• I have tested my code on the test network.

[Fabricevladimir]

Thanks @cgero-eth. Good to go. Why opt for per component files as opposed to a singular z-index pseudo-token dedicated CSS file? I get the separation of concerns, but it makes it quite a bit harder to chase down all the individual custom variables, on the other had, works great for when you're trying to determine whether a particular component has customizable z-index.

[cgero-eth]

Thanks @cgero-eth. Good to go. Why opt for per component files as opposed to a singular z-index pseudo-token dedicated CSS file? I get the separation of concerns, but it makes it quite a bit harder to chase down all the individual custom variables, on the other had, works great for when you're trying to determine whether a particular component has customizable z-index.

Good point, I thought that in the future we might have multiple customisations per component, we might make colors, background, texts customisable for each component. In that way we'll have all the customisations for one component in a single file, even though it looks pretty scattered right now as we only have z-index customisations