Added a status bar showing the currently open file.
Added file path caching to FileReader.
In SourceCodePanel, replaced a ClearAll/AddText pair with SetText.

Profiling has shown that the number of calls to methods on
StyledTextCtrl should be minimized. They are slow.
Move optimizations in the GUI callback.

Not setting the default text on every change.
Added wrapper over SetTextStatus that avoids setting the same text
again.