Begun with SourceCodePanel.

Will show the source code for the selected error.
Adding a SourceCodePanel to the main application window.
Added item selected callback to TreePanel.

Users can register a callback that will be called when an item with
accossiated Python data is selected. The data is passed as argument.
Tree item selected callback in Hvergelmir.py

Seaches for a call stack in the received data, gets the file name from
that and reads the file. Not doing anything with the contents yet.
Added new dependencies to list in README.md.
Helper class for disk reading.

Can search for files in a set of directories and read found text files.
Using FileReader instead of inline file reading methods.
Doing proper path joining when searching for a file.
Included argument parsing.

The user can now specify --path to choose where source files should be
read from.