DHiDPIHelper has nothing to do with dtkwidget. Move the only function
loadNxPixmap to Dtk::Gui::DIcon. Deprecate the implementation in
dtkwidget.
Log: deprecate dhidpihelper
* remove redundant "dtkwidget" in include directory;
* improve cmake, modify some variable names, use file() to collect
source files for scalability;
* add full target_link_libraries support.
Log: add full target_link_libraries support
Influence: all projects who include headers directly by path