eeschema: code cleaning. SCH_ITEM class used for all schematic items in eeschema. Files reorganization.

Fixed Bug 1829931: calculation of zoom level in Window_Zoom(), so we show more rather than less for a box-zoom.

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+eeschema
        * commiting my changes to allow multiple instances of a given schematic file within 
a hierarchy:
        ** internally, m_currentScreen has been replaced with m_currentSheet,
                which is a list or 'path' of screens.  The path of screens is used to 
generate
                a series of timestamps, which is converted to flat component reference via 
a look-up
                table in the schematic files.
        ** this means that m_currentScreen is no longer used -- use GetScreen().
        ** GetScreen is virtual, as some of the dialogs keep around a WinEDA_BaseScreen 
pointer.
        ** all sub-sheets in a given schematic must have different names to generate a 
meaningful netlist.