Tektronix graphics support

**Is your feature request related to a problem? Please describe.**
This is not related to any problem I've found, but from a completeness point of view, it'd be useful to describe graphical instructions for supporting terminals such as xterm or the Digital VT-x3x or x4x series.

**Describe the solution you'd like**
Capture the mode change command (to enter and leave Tektronix mode) and switch to displaying descriptions for the Tek 401x protocol.

**Describe alternatives you've considered**
None, since this would be an extension of the functionality.

**Additional context**
There is at least one other graphics description language that might be worth looking into - ReGIS. Any support for multi-mode protocols would be wise to include space for expansion.