New explorer component

src/sakia/gui/graphs/explorer/__init__.py

src/sakia/gui/graphs/explorer/controller.py

src/sakia/gui/graphs/explorer/edge.py

src/sakia/gui/graphs/explorer/explorer.ui

src/sakia/gui/graphs/explorer/graphics_view.py

src/sakia/gui/graphs/explorer/model.py

src/sakia/gui/graphs/explorer/node.py

src/sakia/gui/graphs/explorer/scene.py

src/sakia/gui/graphs/explorer/view.py

src/sakia/gui/graphs/explorer_tab.py

src/sakia/gui/graphs/views/__init__.py

src/sakia/gui/graphs/views/edges/__init__.py

src/sakia/gui/graphs/views/nodes/__init__.py

src/sakia/gui/graphs/views/scenes/__init__.py

src/sakia/gui/informations/controller.py

src/sakia/gui/navigation/controller.py

src/sakia/gui/navigation/model.py

src/sakia/gui/txhistory/model.py