Toolbar Button: Handle click instead of mousedown

Revert [c4b9d5b][]

Requiring mouse events to trigger `<button>` element event listeners
poses accessibility issues for keyboard users.

The context in the original commit ([c4b9d5b][]) is fairly light, and
there doesn't appear to be a link to a GitHub pull request with more
information. The provided context is:

> Prevents flickering of the placeholder text when clicking block
> formatting buttons on an empty document.

Since the commit is from 2014, there is hope that the underlying flicker
might have been resolved through the natural course of browser
improvements and device enhancements.

The benefit of responding to keyboard events when the `<button>` element
has focus outweighs the potential downsides.

[c4b9d5b]: c4b9d5b

src/test/test_helpers/toolbar_helpers.js

@@ -5,7 +5,7 @@ import { delay, nextFrame } from "./timing_helpers"
 export const clickToolbarButton = async (selector) => {
   selectionChangeObserver.update()
   const button = getToolbarButton(selector)
-  triggerEvent(button, "mousedown")
+  triggerEvent(button, "click")
   await delay(5)
 }
 

src/trix/controllers/toolbar_controller.js

@@ -31,12 +31,12 @@ export default class ToolbarController extends BasicObject {
     this.actions = {}
     this.resetDialogInputs()
 
-    handleEvent("mousedown", {
+    handleEvent("click", {
       onElement: this.element,
       matchingSelector: actionButtonSelector,
       withCallback: this.didClickActionButton,
     })
-    handleEvent("mousedown", {
+    handleEvent("click", {
       onElement: this.element,
       matchingSelector: attributeButtonSelector,
       withCallback: this.didClickAttributeButton,