Map: generate pixmap name in model

Experimentation has shown that the image of a flag will
only be changed after dataChanged() if it is a simple
property. The old code had a complex QML expression and
then - for some reason - it didn't work.

To give us better control over the flags and avoid full
reloads of the map therefore introduce a model-property
pixmap name. The name depends on whether the site is
selected and if not, whether we are in divesite-edit mode.
This makes the code rather convoluted. Firstly, we have
to save whether the site is selected in the map-item.
Secondly we have to access the global map-widget, which
in turn has to go to the map-widget helper (layering
violation!).

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

map-widget/qmlmapwidgethelper.h

@@ -37,6 +37,7 @@ public:
 	Q_INVOKABLE void selectVisibleLocations();
 	Q_INVOKABLE void selectedLocationChanged(struct dive_site *ds);
 	QString pluginObject();
+	bool editMode() const;
 
 private:
 	void updateEditMode();