ListStrEditor behaviour discrepancies between qt and wx versions

`ListStrEditor` behaviour differs in quite a few places between qt and wx implementations:

1. Single select editor initial selection:
- QT: No initial selection
- WX: First item is selected
2. Single select editor, setting `selected_index=-1`:
- QT: Selection is cleared
- WX: All items are selected visually, but `selected` isn't updated
3. Multi select editor, invalid value handling when assigning to `multi_selected`:
- QT: Any invalid values that are assigned to `multi_selected` are ignored during the rest of the update
- WX: No other updated happens
4. Updating editor, both single select and multi select:
- QT: Uses values of `selected` or `multi_selected` as the source of truth for the update
- WX: Uses values of `selected_index` or `multi_selected_indices` as the source of truth for the update
5. Context menu, right click handling
- ~QT: Supports the addition of a context menu, but doesn't register the right clicks via `right_clicked` and `right_clicked_index`~ (Fixed in #1115)
- WX: Doesn't support the addition of a context menu, but does register the right clicks via `right_clicked` and `right_clicked_index`
6. `setx` and `callx` methods only exist in QT editor

It's quite likely that there are even more differences between the implementations hiding in the code.