Add support for use of decoder pro roster icons instead of text for buttons.

[DrCodeWizzard]

As an end user it would be great if EngineDriver supported use of graphical button icons set in DecoderPro Roster. It would prevent shortening of function names in the throttles.

This request is related to the following issue:

JMRI/JMRI#5943

As result of the earlier request I have created and uploaded an initial set of over 380 64x64 png icons that could be used for depictions of functions on sound engines.

Here's are examples of how of how a steam engine with 11 functions is graphically represented in Z21 app, JRMI Throttle, JRMI roster. I am also attaching a copy of that roster.

exampleroster.zip

[mattharris]

duplicate of #365