cleanup: make device code more consistent with core

We keep track of device, i.e. distinct dive computers with id in the core.
The corresponding code stuck out like a sore thumb. Firstly, because it
is C++. But more importantly, because it used inconsistent nameing conventions.

Notably it defined a "DiveComputerNode" when this is something very different
from "struct dive_computer", the latter being the dive-computer related
data of a single dive.

Since the whole thing is defined in "device.h" and the function to create
such an entry is called "create_device_node", call the structure "device".
Use snake_case for consistency with the other core structures.

Moreover, call the collection of devices "device_table" in analogy
with "dive_table", etc.

Overall, this should make the core code more consistent style-wise.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

qt-models/divecomputermodel.h

@@ -26,7 +26,7 @@ slots:
 	void remove(const QModelIndex &index);
 
 private:
-	QVector<DiveComputerNode> dcs;
+	QVector<device> dcs;
 };
 
 #endif