Make screen reader toggable and detect default usage

[rsubtil]

With #221 merged, screen reader will always be used if available. This means it will always be present on Windows and macOS.

For starters, this should be toggle-able in the settings.

But the main issue is figuring out whether this should be enabled or not in the first-time setup. There's no standardized method to detect if a screen reader is currently being used, and the API on all systems works by directly interfacing with the software, even if it's launched or not. Here are the solutions founds so far:

• Make separate builds with and without accessibility plugins enabled
  • Pros: No extra unnecessary dependencies for users that do not need it
  • Cons: Complicates the build process
  • User needs to download the correct version, therefore a source of confusion
  • Does not scale for future optional features; builds become 2^n in quantity for every feature set the user wants.
• Detect running processes for each system
  • Pros: Unified build
  • Cons: Hacky solution, can create false-positives by detecting wrong processes
  • Not implemented in Godot, would require writing custom code
  • Need to investigate process names and software for every major OS
• Enable by default, add option to disable for users
  • Pros: Unified build
  • Works immediately for blind users, where enabling this feature is much harder than for sighted users to disable it.
  • Cons: Sighted users will need to disable it; as it's the majority of users, it's a required step of configuration, breaking RetroHub's "default configuration" policy.
• Design according to industry standards, providing hints for UI elements for screen readers
  • Pros: Industry standard; screen readers work immediately without further modification
  • Cons: Huge programming burden; would need to be implemented in Godot for all systems, as it's custom UI code
  • Doesn't give any indication to the app that screen reader is active, which doesn't allow any changes to it's interface to accomodate better.

[rsubtil]

The most sensible alternative for now is to Enable by default, add option to disable for users.. The first-time wizard speaks a single prompt instructing screen reader users to press a key to enable it.

This will remain open to track if users have issues with the chosen solution.