[Tizen/Desktop] Refactor app un/installation code

This patch contains a big refactoring of the current app
un/installation code. It does the following:
1) all code is encapsulated in the PackageInstaller class
2) Tizen-specific code is encapsulated in the PackageInstallerTizen
class.
3) Generic clean-up and removal of the unused code

This all is necessary as a preliminary step for driving out app
un/installation functionality from XWalk daemon process.