Really do not create con pixmap when not needed

The pixmap of a borderless leaf container will not be used except
for the titlebar in a stack or tabs.

Make sure these containers do not (really) have a pixmap because it can
only get in the way.

fixes #1013
Document the existence of a C ipc project

Add a link to https://github.com/acrisci/i3-ipc which is a new ipc
library in the design phase of development. When it is stable, it will
provide bindings to many high-level scripting languages with
GObject-introspection.

This project aims to replace the unmaintained Python library and offer
an ipc library in new languages such as Lua and JavaScript.
Resize floating windows by right-clicking decoration