Split Project data model from view model:

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Currently we create a GladeWidget object for each project object but

GladeProject and gladeui api in general uses the very same runtime object.

Ideally we should only depend on GladeWidget and the actual runtime object

should only be used for the view in the workspace.

A first step could be updating GladeProject to only use GladeWidget internally

and then we can update gladeui api to use GladeWidget instead but this means a

API/ABI break 

GtkBuilder unsuported features:

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  * No ID objects: GtkBuilder now support objects without an id.

  * Property bindings

Unsupported object classes:

==========================

Glib

  * GMenu/GMenuModel (<menu> GtkBuilder element)

    Because GMenu is in Glib library it can not implement GtkBuildable iface which

    is in GTK+ this lead to implementing GMenu object construction in GtkBuilder

    using a custom element <menu>

    Ideally we should move GtkBuilder and GtkBuildable to Glib and rename them

    GBuilder and GBuildable so that we can implemet GBuildable in GMenu object

    A way to avoid this would be to create a new object type in GTK that derives

    from GMenu say GtkMenuObject (GtkMenu is already taken ;) and make it implement

    GtkBuildable iface.

  * GAction, GSimpleAction, GActionGroup

GTK+ 3.4

  * GtkApplication (add buildable iface to support GMenuModel?¿)

GTK+ 3.14

  * type="action" children in GtkDialog

GTK+ 3.16

  * GtkGLArea

GTK+ 3.20

  * GtkShortcutsWindow

  * GtkFileChooserNative