File dialog improvements

- Add a globally-accessible function to handle the parsing of filter extensions
- Remove the ability of putting the wildcard ('*') among other patterns; it's either a list of patterns or a single '*' now
- Add a hint to select between portals and Zenity on Unix

CMakeLists.txt

@@ -2873,6 +2873,7 @@ elseif(N3DS)
 endif()
 
 if (SDL_DIALOG)
+  sdl_sources(${SDL3_SOURCE_DIR}/src/dialog/SDL_dialog_utils.c)
   if(UNIX AND NOT APPLE AND NOT RISCOS AND NOT HAIKU)
     sdl_sources(${SDL3_SOURCE_DIR}/src/dialog/unix/SDL_unixdialog.c)
     sdl_sources(${SDL3_SOURCE_DIR}/src/dialog/unix/SDL_portaldialog.c)