Added Update Filters.py filter

Added a Filter that can update all other filters in the MCEdit installation to most current version.

- Filter creator don't have to comply to the Updater's VERSION and UPDATE_URL variables. If the Updater can't find the variables, it will skip the filter.
- Filter to download is on the creator's side of the updating, so malicious filters cannot be downloaded easily.
- Supports filters with the version in the filename in addition to the variable.
- The creator can also supply a change log, and the user can view it if they select the "View Change Logs" option.
- If the filter is already at the current version, the Updater will move on to the next filter.

[Here is a GIST of what is required for a filter to be updated with this filter](https://gist.github.com/Podshot/4b2a297a30c0e24243cb)