[Big Picture Mode] Clarify Button Controls for the Steam Web Browser

[Neko-san]

ISSUE
Big Picture Mode is meant for controller users but the Big Picture Mode web browser doesn't do enough to clarify its controls for these users, so using it for these users can be frustrating because most of the time nothing is labeled, and for the times that button controls are stated, they are usually hidden quickly and for most of the time.

Additionally, there is no menu to review what each button does for the Steam Big Picture Mode web browser nor a way to reconfigure what these buttons do for users that would desire to do so.

OS
Windows 10.0.19042 Build 19042

STEPS
- Start Big Picture Mode
- Using a controller, open the Big Picture Mode web browser
- Attempt to use the web browser as you normally would but exclusively with the controller and only use a keyboard explicitly when necessary (such as if the virtual keyboard is too difficult to use for entering a URL)

RESULTS
For less experienced users, the Big Picture Mode web browser can be a pain to navigate due to a lack of clarity for what controller buttons do what in the UI.

DESIRED
The Big Picture Mode web browser UI should more clearly show what controller buttons do what and should offer a menu to view and change these bindings as the user sees fit.