Only allow one instance of Windows installer

Ported from linux\win\installer.nsi
Uninstall previous versions before installation

Ported from linux\win\installer.nsi
Make D2 installation mandatory on Windows
update Visual D version to 0.3.38-1
Merge pull request #83 from eco/port-linux-nsis-features

Port Windows installer features from linux\win version
Merge pull request #84 from rainers/visuald_0.3.38-1

update Visual D version to 0.3.38-1
Merge pull request #82 from jordisayol/master

Add system menu entry for an easy access to dmd/phobos documentation (deb package).
Update dmd_deb.sh

Fix Issue 13047 - cannot stat `./icons/16/dmd-source.png'
Merge pull request #85 from AndrewEdwards/master

Update dmd_deb.sh