Better interaction with plugins in UI

[vbmacher]

Currently, regarding UI - plugins can have their own UI windows. In addition, a compiler can compile directly into a memory and CPU controls debug panel. I think it might be interesting to let plugins influence other UI controls, like:

• A plugin can be used to open/save files from "virtual" filesystem (e.g. MITS-88-DCDD could open files from CPM image into editor and save it into the image)
• CPU plugin could "decompile" a memory range into source code editor
• etc.