Fix issues with loading images with alpha disabled.

These issues are fixed:
* loadPNG gave RGBA surfaces unconditionally. Now it gives RGB surfaces if
  its second parameter, defaulting to true, is false. This fixes adding per-
  surface alpha values, because an RGBA surface ignores its per-surface value.
* Surface::loadImage is updated to receive a third parameter, defaulting to
  true, to determine whether alpha is loaded.
* SurfaceCollection::defaultAlpha was never used.
* SurfaceCollection::defaultAlpha defaulted to false, so even if it were used,
  it would have loaded images without alpha.

Selector previews are now shown with an alpha value of 128. This is so that:
* some of the background is still visible, which includes the name of the
  application for which the selector is being called, its description, and
  text in the status bar;
* the preview, if bright, does not obscure the file names too much.

Selector previews are now shown full-screen (320x240) instead of being a
160x160px square on the right.
Load previews from the previews/ subdir of the current dir in selectors.

This does away with per-link selector directories in link files. It is assumed
that, if a user has access to write files to be launched by an application at
some location, he or she also has access to write files in the previews/
subdirectory under it.