libhawaiishell: Add shell property to corona

With this property all the other elements will know which shell
is currently loaded.
client: Set shell handler identifier

Set shell using corona.
client: Save containment settings

Issue: #129
libhawaiishell: Fix typo with lookandfeel type

Fix typo.
client: Set meaningful context property for panels

Setting the "panel" property instead of "view" feels more natural.
desktop: Move configuration button back to the panel

Having a tool box plugin was a mistake, the configuration window was
supposed to be created from these plugins but since it need to know
view specific information it's now created by the panel or desktop
itself.

This leaves the tool box package to contain just a button that we now
move to the shell implementation just like it was meant to be at the
beginning.

Another valid reason to have a tool box package could have been
support for different platforms (desktop, tablet, ...) and that can
be even easier if the button is part of the shell package.

Issue: #143
Convert all metadata to JSON

JSON files are easier to handle because we don't need extra code from
libqtxdg to handle them (Qt 5 already provides an API).

The only reason I was reluctant and started with desktop entries was that
Transifex supports them, but we can synthesize a desktop file from
the JSON metadata with only Name and Comment key and give it to
the translation system. Such a script would also add the translations
to the JSON files as well.

Issue: #144
translations: Translate metadata

Issue: #145
libhawaiishell: Load metadata from JSON files

Follow latest changes and load JSON files instead of desktop entries.
Since the "mainscript" key is available to all plugin types we
also add a PluginMetadata::mainScript() method.

Issue: #144
client: Use the more appropriate PluginMetadata::mainScript()

PluginMetadata now has a method that returns the main script relative
path, so use that.

Issue: #144
client: Load JSON metadata instead of desktop entries

There are a few places where we don't use packages yet so we have to
manually load the metadata.

Issue: #144
translations: Add scripts to update translations

Issue: #145
translations: Add panel containment metadata Italian translation

Add the first translation to test the scripts.

Issue: #145