fix(toolbar): style all slotted content in order not only buttons (#16617)

- style all slotted content to order properly in a toolbar, not only buttons
- removes duplicated sass variables
- use current color in select so icon will inherit toolbar color

BREAKING CHANGES

- Removes `--icon-color` and `--placeholder-color` from select in favor of inheritance with opacity

core/src/components/segment/test/bug/index.html

@@ -1,16 +0,0 @@
-<link href="../../../../../css/ionic.bundle.css" rel="stylesheet">
-<link href="../../../../../scripts/testing/styles.css" rel="stylesheet">
-<script src="../../../../../dist/ionic.js"></script>
-
-<ion-segment>
-  <ion-segment-button value="all" checked>
-    <ion-label>
-      All
-    </ion-label>
-  </ion-segment-button>
-  <ion-segment-button value="favorites">
-    <ion-label>
-      Favorites
-    </ion-label>
-  </ion-segment-button>
-</ion-segment>