Added ability to customize context icons (per context)

### What does it do ?

Adds ability to define the contexts icon (resource tree) per context.

### Why is it needed ?

As of today, we can customize context icons using `mgr_tree_icon_context` system setting.
However, this setting applies to all contexts.
This PR should allow per context icon, creating `mgr_tree_icon_context` context setting.

### To do before merge

- [ ] create appropriate lexicon strings to explain the system setting