Rename object selection functions in map document.

In the map document we have the following functions:

```
setSelectedObjects // wants MapObject
selectedObjects // returns MapObject
currentObject // returns Object
setCurrentObject // wants Object
currentObjects // returns Object
setTileSelection // relates to selection on the map
tileSelection
setSelectedTiles // relates to selection on the tileset dock
selectedTiles
```

So I'm thinking that some of these should be more specific in their names. For instance `selectedObjects` should be called `selectedMapObjects`. This might change a lot of stuff though, and it's not that bad.

I'm mostly concerned with the tile selection functions I think it makes sense that `tileSelection` returns the selected area on the map rather than the area from the dock. But I'm not sure on what to call `selectedTiles`.

Start by locating the map document declarations and all usages of setSelectedObjects, selectedObjects, currentObject, currentObjects, tileSelection, and selectedTiles. Clarify which functions refer to map selection versus tileset-dock selection, then define consistent names and update every use while preserving their existing return values and inputs.

Written by the indexing model from the issue text.