Add Timer args to struct and add show method

[IanButterworth]

Makes it possible to access & see the initial setup of a Timer.

This PR

julia> t = Timer(1, interval=2)
Timer (open, timeout: 1, interval: 2) @0x000000010c35be50

julia> wait(Timer(t -> println("$(t.timeout) seconds have passed"), 2))
2 seconds have passed

Master

julia> t = Timer(1, interval=2)
Timer(Ptr{Nothing}(0x0000600000b15800), Base.GenericCondition(Base.Threads.SpinLock(0)), true, false)

julia> dump(t)
Timer
  handle: Ptr{Nothing}(0x0000600000b15800)
  cond: Base.GenericCondition{Base.Threads.SpinLock}
    waitq: Base.IntrusiveLinkedList{Task}
      head: Nothing nothing
      tail: Nothing nothing
    lock: Base.Threads.SpinLock
      owned: Int64 0
  isopen: Bool true
  set: Bool true

[JeffBezanson]

I think we should store this as the computed UInt64 milliseconds. It can be printed in seconds with 3 decimals if you like. Not really worth introducing a type instability for this.

[JeffBezanson]

It turns out libuv has an accessor for this, uv_timer_get_repeat https://docs.libuv.org/en/v1.x/timer.html#c.uv_timer_get_repeat, so we can use that instead of storing it.

[IanButterworth]

I tried this but it won't work if the timer has been closed

julia> t = Timer(1, interval = 0.1)
Timer (open, timeout: 1.0 s, interval: 0.1 s) @0x000000010cada9b0

julia> close(t)

julia> t

[91257] signal 11 (2): Segmentation fault: 11
in expression starting at REPL[4]:1
uv_timer_get_repeat at /workspace/srcdir/libuv/src/timer.c:120
getproperty at ./asyncevent.jl:126 [inlined]
show at ./asyncevent.jl:135
show at ./multimedia.jl:47

So I've stored both as their ms values.