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
Merge branch 'fix-tray-mapped'
Merge branch 'master' into next
Use #pragma once

 #pragma once is safer and simpler. According to Wikipedia it's supported by all major compilers.
Snap pointer to resize bar on drag resize

When the user initiates a drag resize, draw the resize bar on the border
of the two involved containers and snap the pointer.

This solution produces cleaner code than the former approach where the
caller obfuscated the click coordinates of the event. This may confuse
someone expecting a true button press event.

Fixes an issue where the resize cursor is not shown when the resize bar
is clicked until the user begins to drag the mouse.

Fixes an issue where focus is not properly updated after the drag is
complete when `focus_follows_mouse' option is set, leaving the pointer
in an unfocused window in some cases.

Fixes an issue where the resize bar may jump a few pixels when the mouse
is first moved.

(Thanks to pbos for suggesting this fix and providing an example
implementation)
Size resizebar according to container size.
i3bar: Don't start child unless status_command

If a command is passed to `start_child` which is NULL, such as in the
case when there is no `status_command` specified in the bar config, do
not start a child process to listen on stdin.

fixes #1140