settings_ui: Add pickers for theme and icon themes (#40829)

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.

crates/onboarding/src/onboarding.rs

@@ -17,7 +17,6 @@ use ui::{
     Divider, KeyBinding, ParentElement as _, StatefulInteractiveElement, Vector, VectorName,
     WithScrollbar as _, prelude::*, rems_from_px,
 };
-pub use ui_input::font_picker;
 use workspace::{
     AppState, Workspace, WorkspaceId,
     dock::DockPosition,