Adaptation to Qt's pimpl idiom design. Improved inheritance safety.

Details:

• Improved unsafe C-style macros inheritance approach to the project-wide

DEFINES: it is unspecified in
which order source files are being preprocessed ones, so in
singleapplication.cpp the class might be inherent of default-specified
base, while in main.cpp this can be a user-provided QAPPLICATION_CLASS
definition.

• Replace Qt's pseudo-keywords to more library-independent counterparts

('emit -> Q_EMIT', etc)

• Indentation fixes (80 character column-width)

• Rearragned #include statements: from most platform-independen to more

app-specific.

• Fixed some grammar typos.

• README.md updated respectively.