fix DTD: add support orientation="staggered"
Merge pull request #676 from fzzrr/fix/dtd

fix DTD: add support orientation="staggered"
Updated the DTD file included in libtiled-java

This updates the file to the latest version. However, the latest version
of the DTD is still missing years of additions.

Closes #674
Fixed terribly slow object movements with cursor keys

It had not been adapted yet to the switch to storing object positions in
pixels rather than tiles.
tmxviewer: Support staggered maps
tmxviewer: Added error handling

* Print out the error message
* Don't crash when the map failed to load