Fix "Insert Text" keyboard shortcut

[aude]

The selector for the "Insert Text" keyboard is not specific enough, it focuses the wrong textbox some times (for example when having the "search" feature open in a group chat).

• I've made the selector more specific.
• I also renamed "Insert Text" to "Focus Textbox" Add Attach files menu item and keyboard shortcut #1014 (comment)
  • I chose "Focus Textbox" over "Focus Text Input" for brevity. Can rename if wanted.

[sindresorhus]

I would prefer Focus Text Input.

[aude]

Okay, I've updated the PR.