Added support for reinstallation and downgrades

This pull request extends client library for new transaction flags.

It adds `--allow-reinstall` and `--allow-downgrade` options to `pkcon` client tool.

It adds two policy actions:
 * `org.freedesktop.packagekit.package-reinstall`
 * `org.freedesktop.packagekit.package-downgrade`

And adds support for these operations to hif backend. Others should keep their current behaviour, e.g.: `error: package xyz is already installed`. Until they are tuned by their maintainers.