RFR: 8262725: IGV: crash when removing all graphs in a group [v2]

[Roberto Castañeda Lozano]

> This change makes IGV close graph views when their groups are either a) removed or b) emptied (all graphs are removed but the group remains), and avoids faulty graph view computation in case b).
> 
> Tested the following scenarios manually (on groups loaded both via network and from a file, and on JDK 8, 11, and 15):
> 1. open a graph, then remove all graphs in the group (as described in the [bug report](https://bugs.openjdk.java.net/browse/JDK-8262725));
> 2. open a graph, then remove its group; and
> 3. open a graph, then remove all graphs and groups (File -> Remove all graphs and groups).

Roberto Castañeda Lozano has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Simplify listener registration
 - Merge branch 'master' into JDK-8262725
 - Close graph views when their groups are removed or emptied

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3540/files
  - new: https://git.openjdk.java.net/jdk/pull/3540/files/955a01c5..691b51cc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3540&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3540&range=00-01

  Stats: 29523 lines in 962 files changed: 2466 ins; 23481 del; 3576 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3540.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3540/head:pull/3540

PR: https://git.openjdk.java.net/jdk/pull/3540