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

include/SDL3/SDL_dialog.h

@@ -37,7 +37,9 @@ extern "C" {
  * `name` is a user-readable label for the filter (for example, "Office document").
  *
  * `pattern` is a semicolon-separated list of file extensions (for example,
- * "doc;docx").
+ * "doc;docx"). File extensions may only contain alphanumeric characters,
+ * hyphens, underscores and periods. Alternatively, the whole string can be a
+ * single asterisk ("*"), which serves as an "All files" filter.
  *
  * \sa SDL_DialogFileCallback
  * \sa SDL_ShowOpenFileDialog