fixing 'edit host configuration' menu item

[rerpha]

The exceptions occurred due to the finalizer method of the BlocksMenu class trying to remove a listener which had already been disposed of if the menu was not open.

There is a slight difference in the way that the 'edit host configuration/component' menu item is now greyed out instead of invisible when a user does not have write access to a block, though I think this is more clear.

[Tom-Willemsen]

The exceptions occurred due to the finalizer method of the BlocksMenu class trying to remove a listener which had already been disposed of if the menu was not open.

This isn't quite true - the exceptions occured due to attempting to use a menu (via getMenuItems) from a listener while that menu isn't open - causing a widgetDisposed error, since the menu has been disposed of by eclipse.

There was a separate issue relating to the finalizer, which would never have run because the listener itself has a reference back to this, and therefore the class would previously have leaked listeners (and therefore memory). This is fixed now by only ever adding one listener, not one listener each time the menu is opened. Note that finalize usage is pretty much always wrong except when doing things with native methods or WeakReference or other very specialized uses.

[JackEAllen]

Looks good and works as expected. I can't edit the host configuration on another instrument as to be expected, but can on my own dev instrument setup.

I have left a few nit-picks which I would like to get your opinion on.

[JackEAllen]

When this issue is ready for merging, The changes should be squashed and merged to changes are easier to cherry pick into release.

[JackEAllen]

From a quick test, still behaves as expected.

[JackEAllen]

LGTM! 👍