Trigger events manually if setting model attribute does not

[philippjfr]

The need for this fix arises from the difference in the way the comm based syncing and bokeh server work. On bokeh server bokeh model properties are always synced between frontend and backend, which means that if a value changes it knows whether that value has changed. When communicating via comms no such syncing occurs and we have to trigger a message no matter what.

• Closes widget callback issue #131

[codecov-io]

Codecov Report

Merging #132 into master will increase coverage by 0.2%.
The diff coverage is 94.59%.

@@            Coverage Diff            @@
##           master     #132     +/-   ##
=========================================
+ Coverage   89.68%   89.88%   +0.2%     
=========================================
  Files          27       27             
  Lines        4120     4153     +33     
=========================================
+ Hits         3695     3733     +38     
+ Misses        425      420      -5

Impacted Files	Coverage Δ
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/util.py	79.31% <100%> (+4.9%)	⬆️
panel/viewable.py	70.04% <85.71%> (+0.62%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 82e3f43...1377ce1. Read the comment docs.