Added source files are not aligned correctly in pro-file

1. Create a new Qt Widgets Application project.
2. Open the project's pro-file.
   It contains lines like:

   SOURCES += main.cpp\
           mainwindow.cpp
   

3. Add a source file to the project using "File" -> "New File or Project..." -> "C++" -> "C++ Source File" and the provided defaults.
   The pro-file now contains:

   SOURCES += main.cpp\
           mainwindow.cpp \
       source.cpp
   

These lines should be aligned correctly.