fix: Close unused MessageView when swapping view

Closes https://todo.sr.ht/~sircmpwn/aerc2/379

The old `MessageView` was not closed when replacing the tab content, which
causes a memory leak.

lib/ui/tab.go

@@ -1,6 +1,8 @@
 package ui
 
 import (
+	"io"
+
 	"github.com/gdamore/tcell"
 	"github.com/mattn/go-runewidth"
 
@@ -107,6 +109,9 @@ func (tabs *Tabs) Replace(contentSrc Drawable, contentTarget Drawable, name stri
 		if tab.Content == contentSrc {
 			tabs.Tabs[i] = replaceTab
 			tabs.Select(i)
+			if c, ok := contentSrc.(io.Closer); ok {
+				c.Close()
+			}
 			break
 		}
 	}

widgets/msgviewer.go

@@ -317,8 +317,9 @@ func (mv *MessageViewer) NextPart() {
 	mv.Invalidate()
 }
 
-func (mv *MessageViewer) Close() {
+func (mv *MessageViewer) Close() error {
 	mv.switcher.Cleanup()
+	return nil
 }
 
 func (ps *PartSwitcher) Invalidate() {