VS2019 Edit Project File fix

Gracefully handle exceptions seemingly caused by accessing the currently active document via the "Edit Project File" VS2019 context menu entry.
See issue #41 for details.

Statusbar XPath format setting

Fixes inability to save statusbar XPath format via the settings dialog.
See issue #42 for details.

Query result source tracking

The XPath Runner query results now activate or open the correct source document when clicked.
See #38 for details.

XPath Tools VS 2019 compatibility

Support for VS 2019 and asynchronous package load (required by VS 2019 Update 1) has been implemented.

Unfortunately support for VS 2012 and VS 2013 has been dropped as a consequence.

See #36 for details.