Please add "RelWithDebInfo" to CMAKE_CONFIGURATION_TYPES

[colugomusic]

When using nappgui as a subproject, unfortunately it will override the value of CMAKE_CONFIGURATION_TYPES, which breaks any project which needs to build using the "RelWithDebInfo" config. This is easy to add like so: colugomusic@81b31a2

Although I think nappgui should not be assuming that it is the "top-level" cmake project like this since it makes it more difficult to use as a subproject. The same issue would arise for anyone who wants to use the "MinSizeRel" config.