Debugger variables view should preserve state across breakpoint hits

**Is your feature request related to a problem? Please describe:**
When a breakpoint is hit in the debugger, the variables display resets - all tables are collapsed, and the scroll position is reset. This makes it inconvenient to track how a nested variable changes over time.

**Describe the solution you'd like:**
Track which items are expanded, and reapply that state. I'd expect that this should happen if the same breakpoint was hit, but it might also be useful to apply the is-expanded state to items whose names match in a different file/line number - Visual Studio does this. That would for example allow the easy inspection of the contents of the message table in on_message in different scripts.

**Describe alternatives you've considered :**
Using the history of the console Evaluate Lua pane to inspect the nested value. It's quite fiddly - pressing F5 to continue execution takes focus away from it, so you have to click, press Cursor Up and Return.