Add ui_font_size setting (#3199)

This PR adds a new `ui_font_size` setting that can be used to control
the scale of the entire UI.

We use the value in this setting to set the base rem size of the window.

Release Notes:

- N/A

crates/ui2/src/elements/button.rs

@@ -1,9 +1,9 @@
 use std::sync::Arc;
 
-use gpui2::{div, DefiniteLength, Hsla, MouseButton, WindowContext};
+use gpui2::{div, rems, DefiniteLength, Hsla, MouseButton, WindowContext};
 
-use crate::{h_stack, Icon, IconColor, IconElement, Label, LabelColor};
-use crate::{prelude::*, LineHeightStyle};
+use crate::prelude::*;
+use crate::{h_stack, Icon, IconColor, IconElement, Label, LabelColor, LineHeightStyle};
 
 #[derive(Default, PartialEq, Clone, Copy)]
 pub enum IconPosition {
@@ -151,7 +151,7 @@ impl<V: 'static> Button<V> {
             .relative()
             .id(SharedString::from(format!("{}", self.label)))
             .p_1()
-            .text_size(ui_size(cx, 1.))
+            .text_size(rems(1.))
             .rounded_md()
             .bg(self.variant.bg_color(cx))
             .hover(|style| style.bg(self.variant.bg_color_hover(cx)))
@@ -198,7 +198,7 @@ impl<V: 'static> ButtonGroup<V> {
     }
 
     fn render(self, _view: &mut V, cx: &mut ViewContext<V>) -> impl Component<V> {
-        let mut el = h_stack().text_size(ui_size(cx, 1.));
+        let mut el = h_stack().text_size(rems(1.));
 
         for button in self.buttons {
             el = el.child(button.render(_view, cx));

crates/ui2/src/elements/icon.rs

@@ -1,4 +1,4 @@
-use gpui2::{svg, Hsla};
+use gpui2::{rems, svg, Hsla};
 use strum::EnumIter;
 
 use crate::prelude::*;
@@ -175,8 +175,8 @@ impl IconElement {
     fn render<V: 'static>(self, _view: &mut V, cx: &mut ViewContext<V>) -> impl Component<V> {
         let fill = self.color.color(cx);
         let svg_size = match self.size {
-            IconSize::Small => ui_size(cx, 12. / 14.),
-            IconSize::Medium => ui_size(cx, 15. / 14.),
+            IconSize::Small => rems(0.75),
+            IconSize::Medium => rems(0.9375),
         };
 
         svg()

crates/ui2/src/elements/label.rs

@@ -1,4 +1,4 @@
-use gpui2::{relative, Hsla, WindowContext};
+use gpui2::{relative, rems, Hsla, WindowContext};
 use smallvec::SmallVec;
 
 use crate::prelude::*;
@@ -86,7 +86,7 @@ impl Label {
                         .bg(LabelColor::Hidden.hsla(cx)),
                 )
             })
-            .text_size(ui_size(cx, 1.))
+            .text_size(rems(1.))
             .when(self.line_height_style == LineHeightStyle::UILabel, |this| {
                 this.line_height(relative(1.))
             })