consider refactoring POI icons

[namdre]

icon functionality could be achieved with the existing imgFile POI attribute and some pre-defined images with the following advantages:

• no need for a new attribute / less code
• imgFile for POI supports dynamic coloring with 'color' attribute
• imgFile for POI supports user defined icon size

@angelobanse @palvarezlopez comments?

[palvarezlopez]

@namdre the problem of imgFile is, exactly, the pre-defined images. The objetive of the icon is tho have these predefined images already embedded in the sumo binarie. If the user has to search and put the image in the same folder of simulation, then normally this feature will not be used.

The main objetive of this new feature is the data downloaded from OSM maps, to have a quick representation of objects like trees, signals, etc.

[namdre]

The simulation already loads xsd files from SUMO_HOME/data and we also provide 3D models in data/3D for convenience. It wouldn't hurt to automatically load images from a new data/icons folder (for sumo-gui and netedit).
Making these easily accessible in the netedit attribute dialog is then the only new code necessary.