Add the possibility to customize the src folder name

[dsebastien]

It should be possible for users to select the name of the folder containing the project sources (what we call 'app' by default and maybe 'src' soon)

Related to dsebastien/modernWebDevBuild#82.

If this option is implemented then it should ensure that the build configuration is also set accordingly

Also, any generated configuration file containing references to that folder name should use a variable and the value should be interpolated during generation (just like the app name ...)