Update Platform Controls (#13751)

Continuing from #13597, this PR refactors platform controls to extract a
generic set of platform controls that can be used for any platform that
does not define it's own/we don't use the system ones.

In the future, these controls will likely be used as a fallback on
windows as well when the windows icon font isn't available.

Release Notes:

- Added updated window controls on Linux

crates/ui/src/styles/color.rs

@@ -23,6 +23,7 @@ pub enum Color {
     Selected,
     Success,
     Warning,
+    Custom(Hsla),
 }
 
 impl Color {
@@ -46,6 +47,7 @@ impl Color {
             Color::Selected => cx.theme().colors().text_accent,
             Color::Success => cx.theme().status().success,
             Color::Warning => cx.theme().status().warning,
+            Color::Custom(color) => *color,
         }
     }
 }