UI refinements + Popover (#3377)

[[PR Description]]

- Update default ui_font_size
- Update toolbar style
- Start on ui::Popover

Release Notes:

- N/A

crates/ui2/src/components.rs

@@ -10,6 +10,7 @@ mod input;
 mod keybinding;
 mod label;
 mod list;
+mod popover;
 mod slot;
 mod stack;
 mod toggle;
@@ -30,6 +31,7 @@ pub use input::*;
 pub use keybinding::*;
 pub use label::*;
 pub use list::*;
+pub use popover::*;
 pub use slot::*;
 pub use stack::*;
 pub use toggle::*;