Updated product.wxs and added the possibility to change the install folder

In this pull request I have tried to simplify the installer project. 
I did this because I was using this installer in a project in our company and I found it quite complicated to start with. I hope that this version that I am proposing will look more intuitive also to you. I think that if you have somewhere a sample project that is building a xll addin (32 and 64 bits) I could add it to the project so that the end user will only need to add his GUID and the project will compile.

I also found a couple of bugs/things to add:
* there was a problem related to language localization in the custom action (part of it was already solved in a preceding commit)
* I added the possibility to change the folder where the addin is installed.