Detect background changes to database file.

This gives the option to reload the database.
A setting is available to let the program automatically reload the
database if there is no change, always ignore, or always ask what to do.

Conflicts:
	src/core/Config.cpp
	src/gui/DatabaseTabWidget.cpp
	src/gui/DatabaseTabWidget.h
	src/gui/DatabaseWidget.h
	src/gui/SettingsWidget.cpp
	src/gui/SettingsWidgetGeneral.ui