Use a GtkBuilder template for GsShellUpdates

Move the UI definition to separate file and use a GtkBuilder template
for subclassing the widget.
Use a GtkBuilder template for GsShellDetails

Move the UI definition to separate file and use a GtkBuilder template
for subclassing the widget.
Use a GtkBuilder template for GsShellCategory

Move the UI definition to separate file and use a GtkBuilder template
for subclassing the widget.
Use a GtkBuilder template for GsShellSearch

Move the UI definition to separate file and use a GtkBuilder template
for subclassing the widget.
Use a GtkBuilder template for GsShellInstalled

Move the UI definition to separate file and use a GtkBuilder template
for subclassing the widget.
Use a GtkBuilder template for GsShellOverview

Move the UI definition to separate file and use a GtkBuilder template
for subclassing the widget.