Update equality overloads for C++20

C++20 added new reverse and rewritten candidates. This can confuse the compiler because it'll test both A==B and B==A for overloads.
Because we were defining parent class equality overloads, A==B and B==A was considered ambigious due to both being compatible in casting.

So we needed to add explicit child class equality operator overloads

pcbnew/pcb_tablecell.cpp

@@ -140,7 +140,6 @@ double PCB_TABLECELL::Similarity( const BOARD_ITEM& aBoardItem ) const
     return similarity;
 }
 
-
 bool PCB_TABLECELL::operator==( const BOARD_ITEM& aBoardItem ) const
 {
     if( aBoardItem.Type() != Type() )
@@ -148,9 +147,14 @@ bool PCB_TABLECELL::operator==( const BOARD_ITEM& aBoardItem ) const
 
     const PCB_TABLECELL& other = static_cast<const PCB_TABLECELL&>( aBoardItem );
 
-    return     m_colSpan == other.m_colSpan
-            && m_rowSpan == other.m_rowSpan
-            && PCB_TEXTBOX::operator==( other );
+    return *this == other;
+}
+
+bool PCB_TABLECELL::operator==( const PCB_TABLECELL& aOther ) const
+{
+    return     m_colSpan == aOther.m_colSpan
+            && m_rowSpan == aOther.m_rowSpan
+            && PCB_TEXTBOX::operator==( aOther );
 }