Add "cancel" command in order to cancel print on Prusa Printers (#1414)

The Prusa / Marlin based printers are sending the "action:cancel" (see [1] and [2]) command via serial port. This commit adds the "action:cancel" command support.

[1]: https://github.com/prusa3d/Prusa-Firmware-Buddy/blob/b3ee47aa573f4d9b4af82d7a6963f43ab37ccc08/src/common/serial_printing.cpp#L13
[2]: https://github.com/MarlinFirmware/Marlin/blob/9e879a5b1f801e7572e7948be38a6dad16ad35d8/Marlin/src/feature/host_actions.cpp#L64

printrun/pronsole.py

@@ -1316,7 +1316,7 @@ def recvcb_actions(self, l):
                 command = command.split(":")
                 if len(command) == 2 and command[0] == "action":
                     command = command[1]
-                    if command == "pause":
+                    if command in ["pause", "cancel"]:
                         self.do_pause(None)
                         sys.stdout.write(self.promptf())
                         sys.stdout.flush()

printrun/pronterface.py

@@ -2055,7 +2055,7 @@ def recvcb_actions(self, l):
                 if len(command) == 2 and command[0] == "action":
                     command = command[1]
                     self.log(_("Received command %s") % command)
-                    if command == "pause":
+                    if command in ["pause", "cancel"]:
                         if not self.paused:
                             wx.CallAfter(self.pause)
                         return True