Merge branch 'hotfix/1.7.3'
Support external images in PDF image #1468
Process common attributes in chhead in XHTML #1469
Fix PDF colwith percentage unit bug #1477
Correct path in error message #1475
Fix msgblock output in PDF #1472
Fix index entry bug in PDF using XEP #1423
Merge branch 'hotfix/1.7.4' into develop