Select All Of Object Type

Sorry for all of the requests. Another great feature to add would be to ability to select all objects of `type` on a selected layer. This would be a useful and robust feature in the creation of maps.

(Occasional when I am creating maps I accidental place objects with no type, I usually don't notice them until I go to export the lua version of the map. I then manually remove them even though they aren't read in the game)

The issue names no file, test, or entry point, so first trace how the editor selects objects on a layer. Determine how selecting all objects of a specified type should work, then verify that the resulting selection supports map creation and reduces manual cleanup before Lua export.

Written by the indexing model from the issue text.