Collapse focus/hover/normal ternaries into Style.controlFill helpers

TextField, NumberField, Dropdown, SearchableDropdown, and Toggle all
painted their background, border color, and border width with the same
three-line ternary ladder (`_focused ? focusFill : _hot ? hoverFill :
normalFill`). Five components × three properties × three branches is a
lot of room for one of them to drift from the others when a new state
ever gets added.

Add controlFill / controlBorder / controlBorderWidth on Style and rewrite
each call site as a single binding. No visual change.

shell/Ui/TextField.qml

@@ -49,19 +49,9 @@ TextField {
   bottomPadding: verticalPadding
 
   background: Rectangle {
-    color: root._focused
-      ? Style.focusFillFor(root.foreground, root.accent)
-      : (root._hot
-        ? Style.hoverFillFor(root.foreground, root.accent)
-        : Style.normalFillFor(root.foreground, root.accent))
-    border.color: root._focused
-      ? Style.focusBorderFor(root.foreground, root.accent)
-      : (root._hot
-        ? Style.hoverBorderFor(root.foreground, root.accent)
-        : Style.normalBorderFor(root.foreground, root.accent))
-    border.width: root._focused
-      ? Style.focusBorderWidth
-      : (root._hot ? Style.hoverBorderWidth : Style.normalBorderWidth)
+    color: Style.controlFill(root._focused, root._hot, root.foreground, root.accent)
+    border.color: Style.controlBorder(root._focused, root._hot, root.foreground, root.accent)
+    border.width: Style.controlBorderWidth(root._focused, root._hot)
     radius: Style.cornerRadius
   }
 }