Fix CMake package for HF1HawaiiShell

Use a relative path to CMAKE_INSTALL_PREFIX, otherwise path are
not resolved correctly.
libhawaiishell: Support for preference packages

Add a new package type for preferences.
libhawaiishell: Fix CMake package configuration

Rename HawaiiShell_INCLUDE_DIR to HawaiiShell_INCLUDE_DIRS for
consistency.

Also fix the path.
libhawaiishell: List plugins

Add an API to list plugins
translations: Import translations from Transifex
translations: Add extra fields to desktop files

So that we can merge all translated desktop entries and create JSON files
but to do so we first need these extra keys.
libhawaiishell: Improve metadata format

Make it more readable.
Update metadatas

Convert desktop files to JSON.
Move desktop files to data/

These are not translations, rather input to the translation system
hence we move them to data/.
Add backgrounds metdata to the translations
Update backgrounds metadata
Add desktop containment metadata to the translations
Update desktop containment metadata
libhawaiishell: Remove preferences directory definition for backgrounds

There's no need to use files on that directory.
declarative-core: Fix PackagesModelItem

Inherit from QObject otherwise it won't be usable from QML and fix
the filePath() invokable method by changing the first argument to
QString which is known by Qt.

While we are here we rename the properties to match those from
the plugin metadata.