RFR: 8346607: IGV: Support drag-and-drop for opening graph files [v2]

[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" />

Theo Weidmann has updated the pull request incrementally with one additional commit since the last revision:

  Clean up

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23040/files
  - new: https://git.openjdk.org/jdk/pull/23040/files/a011853d..9be4df50

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23040&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23040&range=00-01

  Stats: 117 lines in 1 file changed: 59 ins; 57 del; 1 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