tdf#94485 Lower Calc background spellchecker prio

The Calc spell checker idle handler is currently running in the
REPAINT / MEDIUM priority class - much too high.

This races with the Calc GUI repaint for large documents, like
the one attached to tdf#94485.

So move it to the LOWER priority queue, to fix the lagging GUI.

Change-Id: I6e4b05dd1a977182ec13f7e3a05f0722f502f226
(cherry picked from commit 0db68ef9)
Reviewed-on: https://gerrit.libreoffice.org/19556Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>
(cherry picked from commit be4e0ce8)

sc/source/ui/app/scmod.cxx

@@ -174,7 +174,7 @@ ScModule::ScModule( SfxObjectFactory* pFact ) :
                                         ERRCODE_AREA_APP2-1,
                                         GetResMgr() );
 
-    aSpellIdle.SetPriority(SchedulerPriority::REPAINT);
+    aSpellIdle.SetPriority(SchedulerPriority::LOWER);
     aSpellIdle.SetIdleHdl( LINK( this, ScModule, SpellTimerHdl ) );
     aIdleTimer.SetTimeout(SC_IDLE_MIN);
     aIdleTimer.SetTimeoutHdl( LINK( this, ScModule, IdleHandler ) );