refactor(cluster): Default item rendering to onBeforeClusterItemRendered()

[barbeau]

As discussed in #630, refactor the default cluster item rendering implementation from a private method into the protected method onBeforeClusterItemRendered(). This allows apps that extend DefaultClusterRenderer to easily override the default behavior.

I also made the following style changes to the rendering code to make it easier to read (but doesn't change function):

• Change format of !(item.getTitle() == null) to item.getTitle() != null.
• Re-order 2nd and 3rd clause of IF to make it clearer that the marker title is set to the snippet if the title text isn't available and the snippet is.

Refs: 630

Fixes #630

[codecov]

Codecov Report

Merging #633 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #633   +/-   ##
=======================================
  Coverage   19.00%   19.00%           
=======================================
  Files          71       71           
  Lines        4056     4056           
  Branches      610      609    -1     
=======================================
  Hits          771      771           
  Misses       3251     3251           
  Partials       34       34           

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[arriolac]

LGTM