Make CPDFXFA_App / IXFA_AppProvider saner

Move interface comments from the implementation header to the
interface header.

Replace Create / Release functions with static methods.

Replace dubious Release() methods with deletion via virtual dtor, also
for IXFA_App and IXFA_FontMgr while we're at it.

Untabify and fix (theoretically) illegal _CAP include guard definitions
for fpdfxfa/ headers.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1153553003
diff --git a/fpdfsdk/src/fpdfeditpage.cpp b/fpdfsdk/src/fpdfeditpage.cpp
index f371f82..2fdc7de 100644
--- a/fpdfsdk/src/fpdfeditpage.cpp
+++ b/fpdfsdk/src/fpdfeditpage.cpp
@@ -39,7 +39,7 @@
 			}
 		}
 	}
-	
+
 	CPDF_Dictionary* pInfoDict = NULL;
 	pInfoDict = pDoc->GetInfo();
 	if (pInfoDict)
@@ -49,9 +49,8 @@
 		pInfoDict->SetAt("Creator",FX_NEW CPDF_String(L"PDFium"));
 	}
 
-	CPDFXFA_App* pApp = FPDFXFA_GetApp();
+	CPDFXFA_App* pApp = CPDFXFA_App::GetInstance();
 	CPDFXFA_Document* document = FX_NEW CPDFXFA_Document(pDoc, pApp);
-
 	return document;
 }