subtle bug solved: no response to the first left click command after opening a dialog box (after an item edition, for instance)

pcbnew: Excellon files use only ascii codes in comments (some excellon parsers do not like I18n)

Modified recent change on svn change: "PCB edge always printed when PCB edge layer not selected" was incorrect for eeschema and gerbview
solved bug 2070228

It is resolved by adding 'Exclude Edges_Pcb layer' check box in 'Print' dialog.

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.