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/tools/text/texttool.cpp

@@ -321,4 +321,4 @@ void TextTool::setEditMode(bool b)
 bool TextTool::isChanged()
 {
     return QString::compare(m_text, m_textOld, Qt::CaseInsensitive) != 0;
-}
+}