lazy initialization of highlight pens and brushes for DebugVariableViewer

Constructors wx.Pen() and wx.Brush() require wx.App to exist already.
That causes crash during import of the control,
because import is done before calling main application code.

[TOPOLOGY]
NetName=None
Nodes=0x01
Node2Present=0x01
Node2Name=SlaveNode
Node2DCFName=Slave_0_0.eds
EDSBaseName=eds