Add `Removed()`, `Added()`, and `Modified()` query selectors

[DavidPeicho]

Instead of creating "reactive queries" that sort entities based on added/removed components, I think I will modify the Query class to process queries with extra selectors:

• Added() → query only process the entity when it's the first tick the component is added
• Removed() → query only process the entity when the componet has been removed at this tick
• Modified() → query only process the entity when the componet has been modified at this tick

[DavidPeicho]

Added() and Removed() either need to be implemented by using Command Buffers, or by sending event frm archetypes to queries.

[DavidPeicho]

Removed() and Added() are actually implemented now at the entity level. More info here.