WINDOWS: Flow of taking screenshots on Windows is broken

[salty-horse]

When installing ScummVM with Windows installer, taking screenshots with alt+s always fails silently.

ScummVM's Windows installer installs the program in %PROGRAMFILES%, as it should. This directory requires Administrator permissions, which the installer requests.

The installer also creates shortcuts to run ScummVM. These set the current directory to the installation dir: %PROGRAMFILES%\ScummVM.

Since ScummVM is run without Administrator permissions (which is good), and alt+s tries to save screenshots in the current directory, nothing happens.

I propose the following:
1) Screenshots should be saved in a user-owned directory. I think "%USERPROFILE%\My Pictures" is the common place for those. Alternatively, we can use the directory we already create for save games and configuration: %APPDATA%\ScummVM\Screenshots

2) When alt+s fails to save the screenshot, it should say something. Maybe it should pop up a message using the GUI, or maybe it should write a line to the debug console.

Maybe it already prints an error to stdout, but on Windows 9 I got no console even when running the regular ScummVM shortcut (the one that didn't have "(no console)" in its name). Separate bug?

3) If ScummVM has other non-debugging files it saves to the current directory (that is, files that regular users are expected to create without technical knowledge) we should redirect those to the %APPDATA% dir, if possible.

4) The README doesn't mention where screenshots are saved to at all.

[criezy]

The first point was implemented in PR #927 which was merged in master in commit 3849a3e.
There is also now a warning when it fails to save screenshots when using OpenGL graphics mode (there was already one when using a scaler and not OpenGL).
I also added the possibility for user to specify in the config file the path where screenshots should be saved.
And finally the README has been updated.