Ensure palette layer buttons container is not focusable in electron

When experimental web platform features are enabled, the blink
KeyboardFocusableScrollers feature is enabled.
This causes scrollable container to become keyboard-focusable.
See: https://bugs.chromium.org/p/chromium/issues/detail?id=907284

Even trickier, setting tabindex to -1 on a web component causes
everything inside of it to become untabbable.
See: WICG/webcomponents#774

So the solution in this case is to wrap up the action-group component
with a plain old div with tabIndex -1, and let that div handle the
overflow/scrolling instead.

src/core/components/palette.js

@@ -543,13 +543,15 @@ export class Palette extends LitElement {
         >
           <sp-icon slot="icon">${ChevronLeftIcon()}</sp-icon">
         </sp-action-button>
-        <eomap-action-group
+        <div 
+          tabindex="-1"
           id="layer-buttons"
-          compact
           @scroll=${this.onLayerButtonsScroll}
         >
-          ${this.renderLayerButtons()}
-        </eomap-action-group>
+          <eomap-action-group compact>
+            ${this.renderLayerButtons()}
+          </eomap-action-group>
+        </div>
         <sp-action-button
           class="scroll-arrow"
           quiet