Add a PluginManager class

This class is currently very small but the reason of existence
is to allow subsurface to be easily extendable via plugins.
The current type of plugin that I'm making is Social Network,
but another possibilities:

- Dive Simulation Algorithm
- Import/Export Filters
- Profile Overlays

Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface-core/CMakeLists.txt

@@ -72,6 +72,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	qtserialbluetooth.cpp
 	metrics.cpp
 	color.cpp
+	pluginmanager.cpp
 	${SERIAL_FTDI}
 	${PLATFORM_SRC}
 	${BT_CORE_SRC_FILES}