Support project-wide single-file code generation

[KeyWorksRW]

Description:

VeraCrypt (https://github.com/veracrypt/VeraCrypt) places all of their forms into a single project which then generates a single source and header file for all of the forms. Each derived class must include the entire forms.h even though only one class declaration is actually needed by that class.

While having multiple class declarations in a single header file is not common for class declarations that are expected to be edited, that's not the case for wxFormBuilder generated files which can't be edited. Having a project setting that sets a single file to generate would certainly make converting a project like VeraCrypt easier, and it would also be useful for wxPython and wxRuby projects where using a single file is more common.