add UI to be able to send Partial Update Write Request.

[giuseppe-melis]

Leshan server demo sends ALWAYS PUT (Replace) command to the client when the write operation is performed on the instance:
i.e.:
to update DisableTimeout value to 2000 and NotificationStoring to false:
WRITE \1\1 [5=2000, 6=0] --> leshan uses a PUT (Replace).

User should be able to choose to send a Replace (PUT) or a Write-Partial (POST) Request, through some way into the GUI.

[sbernard31]

#803 add a new button, you can now do a Replace or an Update (Write-Partial).

[giuseppe-melis]

very good!
Thanks Simon