Sectors mode, UDMF: Create Brightness Gradient action can now work in "Light" and "Fade" modes.

Visual mode: added support for "fogdensity" and "outsidefogdensity" MAPINFO properties.
Sector Info panel now shows the number of sector's sidedefs as well as light and fade colors (UDMF only).
UDMF: Sector and sidedef flags are now copied/pasted when using "Copy Properties" and "Paste Properties" actions.
Fixed: in some cases default texture overrides were used even when corresponding option was disabled.
Sector Edit form: light and fade values were not saved when a value was pasted/entered into text box.
Sector Edit form: "reset value" button was not shown for light and fade values when selected sectors had different light/fade colors.
Reverted Open Map form changes from previous commit (my assumption that map marker is always empty was not correct...)
Updated documentation.

Source/Core/Data/DataManager.cs

@@ -1552,11 +1552,7 @@ namespace CodeImp.DoomBuilder.Data
                 }
             }
             currentreader = null;
-
-            if (parser.MapInfo != null)
-                mapInfo = parser.MapInfo;
-            else
-                mapInfo = new MapInfo();
+            mapInfo = parser.MapInfo ?? new MapInfo();
         }
 
         private void loadGldefsFromLocation(GldefsParser parser, string location) {