RFR: 8075: Improve Eclipse development experience

[Christoph Langer]

This PR merely updates the .classpath files to the state they would end up in when you do `Maven -> Update project ...` in current Eclipse.

There were only 2 manual changes:
1. I updated/corrected application/org.openjdk.jmc.flightrecorder.dependencyview/build.properties
2. I removed application/org.openjdk.jmc.browser.attach/.settings/org.eclipse.jdt.core.prefs because it gets generated upon Eclipse import anyway. Unfortunately it does not serve the use as a workaround for deselecting the `--release` compiler option.

In addition to that, I polished the devguide a little bit to hopefully make it more clear and stringent.

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Commit messages:
 - JMC-8075

Changes: https://git.openjdk.org/jmc/pull/489/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=489&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8075
  Stats: 417 lines in 104 files changed: 215 ins; 177 del; 25 mod
  Patch: https://git.openjdk.org/jmc/pull/489.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/489/head:pull/489

PR: https://git.openjdk.org/jmc/pull/489