CompilationDatabaseProjectManager: Add possibility to add build steps

As far as I can see, it is currently not possible to configure build settings for a project loaded from a compilation database ('compile_commands.json') when using the experimental CompilationDatabaseProjectManager plugin.

However, it would be very helpful if it were possible to add these. For many projects, adding a custom command like 'make' is probably enough. Setting custom commands is possible with other project types, but currently not for projects created using the CompilationDatabaseProjectManager plugin.

Being able to specify the build steps directly in Qt Creator would improve the integration, and allow e.g. to just click on build errors to automatically jump to the correct location in the source code.

(tested with current git master, as of commit d6b1e5653bb53a7dd245017ba11dddb6112fc179)