FindSDL: Add missing variables, SDL::SDL and update doc

Frédéric Simonisrequested to merge
fsimonis/cmake:update-findsdl into master

This PR updates FindSDL:

  • It now sets SDL_INCLUDE_DIRS and SDL_LIBRARIES
  • I extended it to create the imported target SDL::SDL
  • I altered the documentation to follow the style used by FindPNG
  • It additionally sets SDL_VERSION, SDL_VERSION_MAJOR, SDL_VERSION_MINOR and SDL_VERSION_PATCH

Fixes: #12911 (closed)
Topic-rename: FindSDL-update

Edited by Frédéric Simonis

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