Modified so that the plugin plots all results

I have modified the QGIS-Gazetteer-Plugin to plot all results on the map, rather than only the one that is selected. The markers are shown in green by default, but whichever marker is selected in the results list is shown in red. Enter or double-click then zoom.

## Advantages
* This approach allows the user to orient themselves and select the most appropriate result from the overview map, before zooming in to the point.

## Limitations and disadvantages
* Moving between results with the mouse or with the up and down key refreshes the canvas, which can take an annoyingly long time. There may be a better way to do this with marker.paint?
* If there are many results, the active marker (red) may not be visible below the inactive markers (green). There's probably a way to change the rendering order?

## Important changes
* I have implemented a result class which takes care of assignment, visibility and setting a marker as active or inactive.
* The row number in the results list is now used as an index to the desired result (and associated marker) rather than searching on the description. **This is important**, since the original gazetteer plugin could go to the incorrect result if there are duplicate names in the list.