tpl/widgets: Clarify widgets' environment variables

The context inside widget templates is now:

- `.` can access all classical site variables (normally including `.Content` and so on). `.Content` refers to the actual page content content.
- `.Widget` is defined as follows:
```
type Widget struct {
	Type       string
	Params     map[string]interface{}
	Identifier string
	Weight     int
	Template   *template.Template
}
```
In particular, you can access `.Widget.Type` (name of widget type) and `.Widget.Params` (config parameters).
- `.WidgetArea` contains the widget collection in the current widget area.
- `.Site.Widgets` contains the collection of all widget areas.

tpl/tplimpl/template_embedded.go

@@ -155,9 +155,10 @@ func (t *templateHandler) embedTemplates() {
 {{- if eq .Name $._wa -}}{{/* Display only the current widget area */}}
 <div class="widget-area widget-area-{{ .Name }}">
   {{- $waname := .Name -}}
+  {{- $wa := . -}}
   {{ range .Widgets -}}
   <div class="widget widget-{{ .Type }}">
-    {{ $context := (dict "$" $.c "wa" $._wa "w" .Type "Params" .Params) }}
+    {{ $context := (dict "$" $.c "WidgetArea" $wa "Widget" .) }}
     {{ partial (print .Type "/widget.html") "widgets" $context }}
   </div>
   {{- end }}{{/* end range widgets */}}

tpl/widget/widget.go

@@ -17,6 +17,7 @@ import (
 	"fmt"
 	"html/template"
 
+	"github.com/spf13/cast"
 	bp "github.com/spf13/hugo/bufferpool"
 	"github.com/spf13/hugo/deps"
 )
@@ -39,9 +40,8 @@ type Namespace struct {
 func (ns *Namespace) Widgets(name string, context interface{}) (interface{}, error) {
 	// Add (_wa: name) index/value to context to access it inside
 	// the embedded template (as Widget Area)
-	outcontext := make(map[string]interface{})
-	outcontext["c"] = context
-	outcontext["_wa"] = name
+	outcontext := cast.ToStringMap(context)
+	outcontext["_wa"] = name // The widget area name
 
 	// See in template_embedded for widgets.html
 	templ := ns.deps.Tmpl.Lookup("_internal/widgets.html")