CRM-21255 - PDFLetterCommon - Detect buttons in new popup style

CRM-21255 - PDFLetterCommon - Detect buttons in new popup style

Overview
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Suppose you enable the extension `org.civicrm.civicase`, set the option
`recordGeneratedLetters=combined-attached`, and create a PDF based on the
instructions in [CRM-21255](https://issues.civicrm.org/jira/browse/CRM-21255).

The PDF generator tries to determine whether the clicked the button labeled
"Preview" or "Download Document". This works correctly with the CiviCase 4 popup
mechanism, but in the CiviCase 5 popup mechanism it doesn't work.

Before
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When you click the "Download Document" button, the operates in a
quasi-preview mode and fails to create any activities. This eventually
leads to a crash because the expected activities are missing.

After
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The controller uses a more accurate check which correctly differentiates
"Preview" and "Download Document" on both CiviCase 4+5.

Comments
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I'm not a huge fan of this patch, but it helps demonstrate the
problem+solution.  @colemanw, perhaps there's a better way to make
the buttons function consistently in CiviCase 4+5?