Win32: Add an UI option to change the GPU backend.
It requires an application restart to work.
Delete some dead and duplicated code
Add a menubar option to change the backend.
It restarts without warning.
Also, disable software rendering when switching, similar to the command-line.
Fix some spacing and tab issues in the resource files.
Merge pull request #6906 from thedax/win32-GPU-UI-option

Win32: Add an UI option to change the GPU backend.