Dispose context before shutting down

Calling System.exit is generally something we try to avoid. Disposing
the context beforehand is kinder and avoids some problems such as
deadlocking during frame disposal.

src/main/java/net/imagej/ui/swing/updater/LauncherMigrator.java

@@ -394,6 +394,7 @@ private void switchToNewLauncher() {
 				return;
 			}
 			// New process seems to be up and running; we are done. Whew!
+			appService.getContext().dispose();
 			System.exit(0);
 		}
 		catch (IOException | InterruptedException exc) {