Use a tree-view on the `VeryGenericIndicator` variable selection

As stated by @kklemens, this dropdown is kind of cryptic right now. It's hard to find what you're looking for there.

A tree-view like below would improve the UX a lot:

```
GLOBAL_POSITION_INT
ALT
HDG
LAT
...
SYSTEM_TIME
TIME_UNIX_USEC
...
```

Start by locating the VeryGenericIndicator variable-selection dropdown and tracing how its available variables are assembled. Compare the current selector with the requested GLOBAL_POSITION_INT and SYSTEM_TIME grouping; done means the cryptic dropdown is replaced by a navigable tree view that groups variables and makes entries easier to find.

Written by the indexing model from the issue text.