Add styling for antarctic ice shelves

Summary:
**Rendering**

Only ice shelves
{F157922}

Ice shelves along with normal glaciers
{F157924}

**Issues/Questions**
For rendering these ice shelves the following tag has been used
<tag k='glacier:type' v='shelf' /> which is a proposed glacier tag(http://wiki.openstreetmap.org/wiki/Proposed_features/Glaciers_tags). This tag is to be used in conjugation with  <tag k='natural' v='glacier' /> i.e both of these tags have to be present so as to express an ice shelf. (however for now the shp2osm converter assigns only the former tag for expressing an ice shelf for correct rendering)
Now to render ice shelves a new visual category has been created(NaturalIceShelf). Since currently there is only one to one mapping between an osm tag(key - value pair) to a visual category, I have mapped NaturalIceShelf to OsmTag( "glacier:type", "shelf" ). However there also exists a mapping for OsmTag( "natural", "glacier" ) to NaturalGlaceir for rendering regular glaciers. It can happen that a regular OSM file containing both of these tags may not get rendered correctly since it may get assigned any styling instead of the right one.
What must be done in such a scenario ? (should a many OSM tags to one visual category map be introduced instead of the current one to one map in OsmPresetLibrary or should things be left as they are for now)

Reviewers: rahn, nienhueser

Reviewed By: nienhueser

Differential Revision: https://phabricator.kde.org/D1996