Mail Panel 2.3.0 beta

• redesigned panel
• added support for viewing attachments (thx @MartkCz)
• added support for viewing mail source code (thx @repli2dev)
• added support for viewing mail in new window (thx @repli2dev)
• added plain text preview as alternative to HTML preview
• added detection of optimal HTML preview height
• previews are collapsed by default and their visibility state can be toggled
• replaced PNG icon with SVG (thx @irkallacz)
• deprecated SessionMailer in favor of FilesMailer
• fixed baseUrl detection
• fixed viewing of plain text-only mails with attachments (thx @repli2dev)
• lots of internal refactoring