1. A computer system, comprising: one or more processors; and one or more computer-readable storage media having stored thereon computer-executable instructions that are executable by the one or more processors to cause the computer system to facilitate an integration of one or more applications with an operating system, and further to cause the computer system to perform at least the following: identify one or more extensible functionalities of an operating system, wherein each of the one or more extensible functionalities is an integrated functionality of the operating system and is controllable at least by the operating system; notify the operating system that at least one of the one or more extensible functionalities of the operating system is serviceable by an application, wherein the notification is communicated to the operating system through one or more entry points of the application; activate the application to service the at least one of the one or more extensible functionalities of the operating system, wherein the application services the at least one of the one or more extensible functionalities of the operating system according to a contract received from the operating system, the contract being received by the application through at least one of the one or more entry points; obtain a contract object through at least one of the one or more entry points of the application, wherein the contract object identifies the contract and specifies how the application is to fulfill the contract; perform processing specified by the contract object to fulfill the contract; and communicate results of the processing to the operating system.