This adds a new PendingInstall event which broadcasts that an install
process has started, but the state of it is not yet known, like
whether it needs to be downloaded still, or is ready to install. It
marks the very first step of the whole InstallManagerService process.
Installer events should only be directly related to the install process as
managed by the Installer set of classes. The newer AppStatusUpdate stuff
now tracks the whole lifecycle of the process.
This mostly reverts f0d6acd974548e24662a64271ae57922f74c3225 since there is
now the overarching concept of "Pending Install" to mark packages that are
somewhere in the whole process.
refs #828
refs #1357
a1edfdfc8d