gui.hpp: Uncomment public header file Button.hpp

* No idea why this was commented out of the header file!

* Remove commented out header file DataViewItem.hpp (duplicate).
Update Joystick class to use SDL2Logger logging categories

* Rename NOM_LOG_CATEGORY_INPUT to NOM_LOG_CATEGORY_EVENT.
* Rename NOM_LOG_CATEGORY_TRACE_EVENTS to NOM_LOG_CATEGORY_TRACE_EVENT.

* tests/src/gui/common.cpp: Update on_click function to use SDL2Logger
logging categories.
Introduce DataViewList unit test

* Remove examples/gui.cpp -- obsoleted by unit tests for the widgets.
* Remove DataViewItem.cpp. This file is not used; DataViewItem is a
template interface.
* Re-enable compiling of DataViewList.cpp.

* Rename DataViewList::RawPtr to DataViewList::raw_ptr.