Msg-explorer

* Fixed bugs in named property viewer.
* Fixed a bug where sorting caused various tables to display incorrectly (forgot to turn off sorting while editing them and turn it back on afterwards).

* Fixed issue that cause the multiple viewer to not clear it's data.

* Fixed issue with files with non-standard streams from custom attachment types. There were issues with viewing the streams in the tree as well as with opening them in the stream viewer.
* Switched `HexViewer` from `QPlainTextEdit` to `QTextEdit` to allow for coloring to easily differentiate the headers from the actual data.
* Added the ability to drag and drop an MSG file onto the window to open it.

* Added `python_requires` to setup.py.

* Added app icon.

* Expanded range of compatible extract-msg versions.
* Fixed bugs that prevented properly viewing properties.