i3bar: Set `mapped` flag on trayclient creation

i3bar: Set `mapped` flag on trayclient creation

When a trayclient is first created as a structure in memory, explicitly
set the `mapped` flag to false. Otherwise it may initialize to `true` in
some circumstances without actually being mapped, causing a request to
be mapped from the client to be ignored.

Create the trayclient in memory before handling a request to be mapped
immediately.

fixes #1138