RFR: 8346607: IGV: Support drag-and-drop for opening graph files

[Theo Weidmann]

Adds the ability to drag-and-drop graph files into IGV.

A welcome screen invites users to drag and drop files:

https://github.com/user-attachments/assets/e08810bd-4e75-42d0-9507-fb24366803f9

The welcome screen disappears while any graphs have been imported (via opening a file or via socket) and reappears automatically if no graphs are imported in the workspace (that is, if the list on the left-hand side is completely empty).

Furthermore, graphs can be dropped at any time in the areas in the green boxes in the image below. The reason only these areas work is due to the unfortunate way in which Apache NetBeans handles drag and drop events, as many components just consume events they cannot process without offering any apparent way to customize the drop behavior.

<img width="2091" alt="Screenshot 2025-01-10 at 16 11 13" src="https://github.com/user-attachments/assets/558d57fd-01f3-4cfb-a70f-a9573cb729e9" />

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Commit messages:
 - Fix whitespace
 - Improve show hide logic
 - Implement file drag and drop

Changes: https://git.openjdk.org/jdk/pull/23040/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23040&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346607
  Stats: 148 lines in 2 files changed: 138 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/23040.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23040/head:pull/23040

PR: https://git.openjdk.org/jdk/pull/23040