RFR: 8297362: EOS might not be delivered by progressbuffer in some cases

[Alexander Matveev]

This is regression from [JDK-8043352](https://bugs.openjdk.org/browse/JDK-8043352). [JDK-8043352](https://bugs.openjdk.org/browse/JDK-8043352) moved clearing pending events in progress buffer when upstream sends EOS. We need to do this for any other events except EOS. If we clear pending EOS, then it will not be delivered and we will hang due to downstream waiting for data or EOS. We still need to clear any other events such as new segment and not deliver it once we receive EOS.

Added "plugins" folder to macOS project. For some reason it was missing and debugging plugins with xcode was not possible.

Tested on all platforms with all formats over FILE and HTTP protocol. EOS was delivered correctly in all cases.

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Commit messages:
 - 8297362: EOS might not be delivered by progressbuffer in some cases

Changes: https://git.openjdk.org/jfx/pull/961/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=961&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297362
  Stats: 20 lines in 2 files changed: 16 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/961.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/961/head:pull/961

PR: https://git.openjdk.org/jfx/pull/961