RFR: JMC-5904: Merge org.openjdk.jmc.ui.celleditors with org.openjdk.jmc.rjmx.ui.celleditors

[Alex Macdonald]

This PR addresses JMC-5904 [[0]](https://bugs.openjdk.java.net/browse/JMC-5904), in which it is noted that org.openjdk.jmc.ui.celleditors could be merged with org.openjdk.jmc.rjmx.ui.celleditors instead of separating the code.

For the most part this is a fairly straightforward migration of code. There are three classes in jmc.ui.celleditors that needed to be moved into rjmx.ui.celleditors:
1. ClearableTextCellEditor
2. CommonCellEditors
3. NullableTextCellEditor

Once these files are moved there is a slight problem: org.openjdk.jmc.ui.misc.FilterEditor uses CommonCellEditors, and org.openjdk.jmc.rjmx.ui requires org.openjdk.jmc.ui. In this case, adding a dependency from org.openjdk.jmc.rjmx.ui.celleditors.CommonCellEditors to org.openjdk.jmc.ui.misc.FilterEditors would introduce a circular dependency.

The solution I've gone with here is to move FilterEditor to org.openjdk.jmc.flightrecorder.ui, which IMO makes sense anyways because it's closer to the code it gets consumed by. FilterEditor is used in:
- _org.openjdk.jmc.flightrecorder.ui_.common.DataPageToolkit
- _org.openjdk.jmc.flightrecorder.ui_.common.FilterComponent
- _org.openjdk.jmc.flightrecorder.ui_.common.LaneEditor
- _org.openjdk.jmc.flightrecorder.ui_.pages.ItemHandlerPage

**Note:** this issue was originally targeted for 8.1.0, but isn't really critical (it's a P4 task), so if desired we can hold off on integrating it now and keep this around for integration into 8.2.0 as discussed in the jmc bi-weekly calls

[0] https://bugs.openjdk.java.net/browse/JMC-5904

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Commit messages:
 - 5904: Merge org.openjdk.jmc.ui.celleditors with org.openjdk.jmc.rjmx.ui.celleditors

Changes: https://git.openjdk.java.net/jmc/pull/271/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=271&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-5904
  Stats: 273 lines in 22 files changed: 93 ins; 172 del; 8 mod
  Patch: https://git.openjdk.java.net/jmc/pull/271.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/271/head:pull/271

PR: https://git.openjdk.java.net/jmc/pull/271