Added ability to customize media source icons

### What does it do ?

Adds ability to customize media source icons (tree) using `iconCls` property.

### Why is it needed ?

Manager customization (Creative Freedom ? :smile:)

### To do before merge

- [ ] create appropriate lexicon strings
- [ ] create a system setting to allow users to define the default class ?
I guess it might be better to also check `!empty($props['iconCls'])`