Mailspring 1.5.4

What's new in this version:

Improvements:
- An important bug that caused Mailspring to occasionally send blank or incomplete drafts (especially long relies or messages with significant quoted text) has been resolved
- Printing email messages now works reliably and a new "Save as PDF" option in the print window makes it easy to export an email thread
- Extremely long messages now clip in the message panel and a "popout" option allows you to view their entire text in a separate window, similar to Gmail. This allows Mailspring to more reliably maintain good performance as you browse your email
- A Brazillian Portuguese translation has been added
- Russian and Japanese translations have been improved
- Mailspring now runs on Electron 4. This should fix the menu bar not appearing in Ubuntu 18.10 (#1258, #1158) and comes with a long list of bug fixes and improvements (Mailspring was previously on Electron 2.0.14.) As of this release, Mailspring and Electron no longer support macOS 10.9
- The has:attachment search filter has been added. Note that this clause only returns messages Mailspring has downloaded and has "seen" the attachments for. (Currently the last three months of mail)

Fixes:
- Sending emails with consecutive whitespaces now works as expected
- Mailspring now adds itself to the Dock in macOS Mavericks properly
- The Send and Archive option is no longer shown when you're composing a new message
- Mailspring no longer hangs when trying to sync a mailbox containing a message with an unidentified MIME part
- Mailspring no longer inserts an extra whitespace when you compose a reply with a default signature
- The tool tip displayed when scrolling in the message panel is now localized correctly
- Copy / paste and the Terms / Privacy links in billing and registration windows now work properly
- Open tracking pixels now display as Sent from Mailspring when the recipient's email client shows image alt text. It's important we have alt text identifying the image in some way for highest deliverability, but the previous open tracking alt text was a bit too revealing for many users