Simple maintenance improvements (#1918)

* Use license file from gnu.org

Downloaded from:
https://www.gnu.org/licenses/gpl-3.0.txt

* Add missing final newline

* Fix link

* Fix shellcheck warnings

* Move donation header one level up

* Fix typos

* Stop mixing tabs and spaces

* Trim trailing whitespace

src/utils/filenamehandler.cpp

@@ -66,7 +66,7 @@ QString FileNameHandler::parseFilename(const QString& name)
  * - If `format` is not given, the suffix will remain untouched, unless `path`
  *   has no suffix, in which case it will be given the "png" suffix
  * - If the path generated by the previous steps points to an existing file,
- * 	 "_NUM" will be appended to its base name, where NUM is the first
+ *   "_NUM" will be appended to its base name, where NUM is the first
  * available number that produces a non-existent path (starting from 1).
  * @param path Possibly incomplete file name to transform
  * @param format Desired output file suffix (excluding an initial '.' character)