Fixes default for project specific compiler settings

Don't reset to default values when disabling project specific compiler settings.
Adds support for boolean settings with default values in the compiler settings hierarchy.

Fixes #1002083

I'm not sure of the `BooleanSettingWithDefault` implementation. I had to copy the code from `BooleanSetting`, because it cannot be simply extended it, it is `scala.tools.nsc` private.
I could make it extend `BooleanSetting` if I was to put the code in a `scala.tools.nsc` package, but I don't know if we want to extends compiler packages inside Scala IDE.