compose: allow setting all headers in templates

Allow setting To, Cc, Subject, Bcc in template headers.

Fixes: https://todo.sr.ht/~rjarry/aerc/19
Signed-off-by: Robin Jarry <robin@jarry.cc>

doc/aerc-templates.7.scd

@@ -26,6 +26,10 @@ Chuck
 If you have a template that doesn't add any header, it *must* be preceded by a
 newline, to avoid parsing parts of the body as header text.
 
+All headers defined in the template will have precedence over any headers that
+are initialized by aerc (e.g. Subject, To, From, Cc) when composing a new
+message, forwarding or replying.
+
 # MESSAGE DATA
 
 The following data can be used in templates. Though they are not all

widgets/compose.go

@@ -109,11 +109,11 @@ func NewComposer(aerc *Aerc, acct *AccountView, conf *config.AercConfig,
 		focused:   1,
 		completer: cmpl,
 	}
-	c.buildComposeHeader(aerc, cmpl)
-
 	if err := c.AddTemplate(template, templateData); err != nil {
 		return nil, err
 	}
+	c.buildComposeHeader(aerc, cmpl)
+
 	c.AddSignature()
 
 	c.updateGrid()