cleanup: use std::string in struct device

struct device is a core data structure and therefore shouldn't use QString.
QString stores as UTF-16 (which is a very questionable choice in itself).
However, the real problem is that this puts us in lifetime-management
hell when interfacing with C code: The UTF-16 has to be converted to
UTF-8, but when returning such a string, this puts burden on the caller
who has to free it. In fact, instead of looping over devices from C-code
we had a callback that sent down temporary C-strings with qPrintable.

In contrast, std::string is guaranteed to store its data as
contiguous null-terminated and C-compatible strings. Therefore,
replace the QString by std::string. Keep the QString just in
one place that formats a hexadecimal number to avoid any
potential change.

The disadvantage of using std::string is that it will crash
when constructed with a NULL argument, consistent with C-style
functions such as strcmp, etc. Arguably, NULL is different
from the empty string even though we treat both as the same.

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

qt-models/divecomputermodel.cpp

@@ -20,9 +20,9 @@ QVariant DiveComputerModel::data(const QModelIndex &index, int role) const
 		case ID:
 			return QString("0x").append(QString::number(node.deviceId, 16));
 		case MODEL:
-			return node.model;
+			return QString::fromStdString(node.model);
 		case NICKNAME:
-			return node.nickName;
+			return QString::fromStdString(node.nickName);
 		}
 	}
 
@@ -59,7 +59,7 @@ bool DiveComputerModel::setData(const QModelIndex &index, const QVariant &value,
 		return false;
 
 	device &node = dcs[index.row()];
-	node.nickName = value.toString();
+	node.nickName = value.toString().toStdString();
 	emit dataChanged(index, index);
 	return true;
 }