settings_ui: Add pickers for theme and icon themes

[danilo-leal]

In the process of adding pickers for the theme and icon themes fields in the settings UI, I felt like there was an improvement opportunity in regards to where some of these components are stored. The ui_input crate originally was meant only for the text field-like component, which couldn't be in the regular ui crate due to the dependency with editor. Given we had also added the number field there—which is similar in also having the same dependency—it made sense to think of this crate more like a home for form-like components rather than for only one component.

However, we were also storing some settings UI-specific stuff in that crate, which didn't feel right. So I ended up creating a new directory within the settings_ui for components and moved all the pickers and the custom input field there. I think this makes it for a cleaner structure.

Release Notes:

• settings_ui: Added the ability to search for theme and icon themes in their respective fields.