preferences: improve category headers

[vince-fugnitto]

Feature Description:

Following #7105, we should improve category headers present in the view.
At the moment, subcategories are styled similarly to categories which makes their appearance confusing. For instance, take the preference markdown.links.openLocation for example:

Theia:

VS Code:

In Theia, it looks like there are two categories (Markdown and Links), and it gives the impression that Markdown is empty.

[liorfr]

Also,

Theia:
The category in the leaf starts from the second category word when the category has more than one word

VS Code:
VS Code Has semicolon after subcategories

[vince-fugnitto]

I believe this can be closed thanks to #9439.

when no search is present:

when a search is present ('markdown')