Do not explicitly set the clipboard text if it already contains the
expected content.
Even if copy-paste loops are avoided by the previous commit, this avoids
to trigger a clipboard change on the computer-side.
Refs #1580 <https://github.com/Genymobile/scrcpy/issues/1580>
After the recent refactorings, a "control event" is not necessarily an
"event" (it may be a "command"). Similarly, the unique "device event"
used to send the device clipboard content is more a "reponse" to the
request from the client than an "event".
Rename both to "message", and rename the message types to better
describe their intent.
