client: border() now mutable.

- When border() changes, on_border is called.
event: Trivial wrapping layout fix.
event: Trivial comment corrections.
Added workaround for some c++14 features when using a c++11 compiler.
client: Added ctor params, make().
client: Updated paramterized ctor test to use ctor params.
events: Implemented parameterized ctor for add/remove client.
event: Removed need for operator == in events.

Originally, this was needed to detect which event was sent through event
target's mock's on_event method. I realized we should be comparing by
object ref, not by value, and updated gtest.