Introduce office POI class (#1423)

Omitted values include:

* office=yes ... too generic
* office=vacant ... not really a POI
* deprecated values (like office=administrative)
* tags that don't have a clear meaning (like office=camping,
  office=healthcare, office=transport, office=design)
* tags where another tag is used more often, e.g:
  office=religion is used more often than office=parish
  office=construction_company is used more often than office=construction
  office=engineer is used more often than office=engineering

---------

Co-authored-by: Tomas Pohanka <TomPohys@gmail.com>

layers/poi/style.json

@@ -743,7 +743,27 @@
             ]
           ]
         },
-        "icon-image": "{subclass}",
+        "icon-image": [
+          "coalesce",
+          [
+            "image",
+            [
+              "get",
+              "subclass"
+            ]
+          ],
+          [
+            "image",
+            [
+              "get",
+              "class"
+            ]
+          ],
+          [
+            "image",
+            ""
+          ]
+        ],
         "text-field": "{name:latin}\n{name:nonlatin}",
         "visibility": "visible",
         "text-anchor": "top",
@@ -778,7 +798,8 @@
           "information",
           "cinema",
           "theatre",
-          "fire_station"
+          "fire_station",
+          "office"
         ],
         [
           "!=",