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

appveyor.yml

@@ -12,7 +12,7 @@ build_script:
     mkdir build
 
     cd build
-    
+
     set QTDIR=%Qt5_INSTALL_DIR%
 
     set "VCINSTALLDIR=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC"
@@ -22,11 +22,11 @@ build_script:
     cmake c:\projects\source -G "Visual Studio 16 2019" -DCMAKE_BUILD_TYPE:STRING=Release -DENABLE_OPENSSL=ON -DRUN_IN_PLACE=OFF
 
     cmake --build . --parallel 2 --config "Release"
- 
+
     cpack -G WIX -B package
 
     mkdir artifact
-    
+
     cp package/*.msi artifact/.
 
     cp src/Release/*.exe artifact/.