Add ServerChannelReorderEvent

[Bastian]

As an addition to the ServerChannelChangePositionListener, it might be useful to have an event that groups all these single events in a single event, that has methods like

• getOldChannelOrder()
• getNewChannelOrder() which return a ordered list.

This might be a helpful reference: https://gist.github.com/Bastian/3ecec76c4101d6866a454df6fae1d1c2

[Saladoc]

Roles should receive a similar event. This goes well with #417