Feature request - small interface improvements

[Bougakov]

Please consider fixing these 2 annoyances in the interface:

• make OpenDACT enumerate all COM ports and pre-select port with largest index on startup. On my system it always chooses hardware port COM1 instead of virtual COM4. Since OpenDACT crashes a lot, choosing correct port every time is annoying.
• please add default value of "G28" to "send GC" input. Please also make the program send the command on "Enter"

Thanks!