sw: fix infinite grammar checking idle loop

The grammar checker always wants to be started in DoIdleJobs(),
even if all paragraphs are already marked as checked.

This is because there is currently no call anywhere of
SwRootFrame::SetNeedGrammarCheck(false) to reset the flag
and prevent DoIdleJobs from trying to start the grammar checker.

This call was already there before but was removed without
any justification in commit 9160fe81.

This has become an infinite loop in several Junit tests with commit
53da556c.

Change-Id: Ibe7ad93442070aac0577725d044281912307d9e8

sw/source/core/doc/docnew.cxx

@@ -172,7 +172,14 @@ bool SwDoc::StartGrammarChecking( bool bSkipStart )
             {
                 bStarted = true;
                 if ( !bSkipStart )
+                {
+                    for (auto pLayout : GetAllLayouts())
+                    {   // we're starting it now, don't start grammar checker
+                        // again until the user modifies the document
+                        pLayout->SetNeedGrammarCheck(false);
+                    }
                     xGCIterator->startProofreading( xDoc, xFPIP );
+                }
             }
         }
     }