Make footprints mostly covered in pads easier to select.

If all the uncovered space is at the very edge it doesn't feel like
there's anywhere to click but the disambiguation menu doesn't come
up.

Fixes https://gitlab.com/kicad/code/kicad/issues/6584

pcbnew/tools/selection_tool.cpp

@@ -2423,7 +2423,7 @@ void SELECTION_TOOL::GuessSelectionCandidates( GENERAL_COLLECTOR& aCollector,
     // Special case: if a footprint is completely covered with other features then there's no
     // way to select it -- so we need to leave it in the list for user disambiguation.
 
-    constexpr double maxCoverRatio = 0.75;
+    constexpr double maxCoverRatio = 0.70;
 
     for( int i = 0; i < aCollector.GetCount(); ++i )
     {