OPI: Disable non-functional controls in OPI context menu

As a user, I don't want access to controls in the OPI context menu that either don't work or worse put the GUI in an undesirable state so that I can be confident that I can work and select controls without breaking the system.

I don't know how easy it is, or whether it's even possible to manipulate the OPI context menu. I did it in the alarm tree view for the E4 migration. We should have a look though.

Things we know work:

- Refresh OPI
- Dump PV list
- Print
- Show PV info
- Clearing graphs
- Show/hide toolbars
- Process variable > log plotter

Debatable:

- Show macros works but opens a console which gives access to a genie_python scripting window.
- Full screen. It does technically work, but you shouldn't affect the whole GUI using the OPI, and you can't use the hotkey to exit full screen. You can only do it by going back into the OPI context menu
- Compact mode, same as above
- OPI screenshot. Quite incredibly the basic functionality works, but the dialog contains several buttons that cause empty error dialogs to appear or do nothing.

Things that don't work:

- Show navigator
- OPI runtime perspective