Heavily revise USnippetExtraHelper unit

1. Moved class that parses old style credits text into active text from
USnippetCreditsParser into the TSnippetExtraHelper.TCreditsParser
private class in USnippetExtraHelper.

2. Removed the now redundant USnippetCreditsParser unit from project.

3. Modify methods of TSnippetExtraHelper that create active text
documents to embed the active text between matched ekDocument elements.

4. Rewrote TSnippetExtraHelper.BuildActiveText to simply hand off the
work by calling to the TActiveTextFactory.CreateActiveText parser
overload, passing the REML parser.

Author: delphidabbler

Src/CodeSnip.dpr

@@ -317,7 +317,6 @@ uses
   USingleton in 'USingleton.pas',
   USnipKindListAdapter in 'USnipKindListAdapter.pas',
   USnippetAction in 'USnippetAction.pas',
-  USnippetCreditsParser in 'USnippetCreditsParser.pas',
   USnippetDoc in 'USnippetDoc.pas',
   USnippetExtraHelper in 'USnippetExtraHelper.pas',
   USnippetHTML in 'USnippetHTML.pas',

Src/CodeSnip.dproj

@@ -519,7 +519,6 @@
 			<DCCReference Include="USingleton.pas"/>
 			<DCCReference Include="USnipKindListAdapter.pas"/>
 			<DCCReference Include="USnippetAction.pas"/>
-			<DCCReference Include="USnippetCreditsParser.pas"/>
 			<DCCReference Include="USnippetDoc.pas"/>
 			<DCCReference Include="USnippetExtraHelper.pas"/>
 			<DCCReference Include="USnippetHTML.pas"/>