CMakeLists.txt maintenance.

* Updates cmake_minimum_requied().
* Gets rid of QTX_LIBRARIES and QT_XXXX_INCLUDE_DIR. It's Qt4 stuff.
* Uses the new Qt5::Module method.
Use the new LxQt::SingleApplication

Allow only one configuration application instance
Make use of QApplication::exec()

Instead of QDialog::exec(). That's the proper way.
Add .gitignore
Add icon to config window
Whole GUI of time/date setup has been rewritten from scratch
the app has now style of LXQT config dialogs based on LxQt::ConfigDialog
the time and date setting is divided into one configuration widget
and timezone selection is divided into second config widget

Licence added into source/header files