Update installation for windows .exe

[huche6]

Description

• No possibility to update an installation with windows .exe installer. It would be nice to have something which would look like the -u flag available for linux .sh installer #679.
• I changed the MessageBox which would poped when a path already exists to a YESNO MessageBox. The message is now "The directory '$INSTDIR' is not empty, do you want to update the installation ?".
  • If the user choose "yes", the installer update the $INSTDIR
  • If the user choose "no", them are send back to select the installation path.
• I added a function in main.nsi.tmpl which is called CheckInstdirValidity. It checks the validity of the selected directory during the installation. It avoids to install something in a windows critical directory or a non conda directory.
• This function was made just by externalising some code from the function un.onInit. I had in mind to do so to be able to call it when necessary. However, for some reasons, I can not achieve to call CheckInstdirValidity within un.onInit. If anyone has some idea about how to do it, it would avoid code duplication.

Checklist - did you ...

• [ ] Add a file to the news directory (using the template) for the next release's release notes?
• [X] Add / update necessary tests?
• [X] Add / update outdated documentation?