Sync SooperLooper loop to itself causes "enable sync" to oscillate

[riban-bw]

Describe the bug
Within SooperLooper, if a loop is synchronised to itself then the "enable sync" controller oscillates on and off.

To Reproduce
Steps to reproduce the behaviour:

1. Add SooperLooper to a chain
2. Set loop 1 "sync to" to "Loop 1"
3. Set loop 1 "enable sync" to "on"

Expected behaviour
"enable sync remains on.

Actual behaviour
"enable sync flickers between on and off.

Setup:

• HARDWARE
  Raspberry Pi 4 Model B Rev 1.4
  Audio: ZynADAC
  Display: ZynScreen 3.5 (v1)
  Wiring: MCP23017_ZynScreen
  I2C: MCP23017@0x20, MCP23008@0x21, ADS1115@0x48, MCP4728@0x60
• SYSTEM
  Raspbian GNU/Linux 10 (buster)
  Build Date: 2020-09-05
  Memory: 6% (126M/1872M)
  SD Card: 70% (19G/29G)
  Temperature: 61.8ºC
  Overclock: Medium
  USB Storage: 5% (9.7G/235G)
• MIDI
  Profile: default.sh
  Tuning: 440 Hz
  Receive Mode: Stage (Omni On)
  ZS3 Sub-SnapShots: On
  MIDI to Output: On
  Master Channel: 16
• SOFTWARE
  zyncoder: testing (6eb5496)
  zynthian-ui: testing (68c0485)
  zynthian-sys: testing (af606bf)
  zynthian-data: testing (45b44f9)
  zynthian-webconf: testing (706d78d)

Additional context
SooperLooper daemon is sending OSC messages for sync on / off. This does not happen with slgui. Maybe it is avoided by logic within slgui, e.g. to avoid setting a loop to sync to itself. Ideally this should be avoided in the SooperLooper process. Reported upstream issue: essej/sooperlooper#39. Could add logic to Zynthian to avoid this situation but that may require some historical state storage to restore to expected state which could add undue complexity. For now add note to docs describing the behaviour.