Cleaning code: moved some help strings used both in toolbars and menus in a single file help_common_strings.h,
in order to avoid slightly different help strings for the same command, and avoid some not needed  translations.

* Continue component library class clean up and encapsulation work.
* Change hierarchical sheet label container to boost::vector_ptr.
* Encapsulate hierarchical label handling in hierarchical sheet class.
* Convert some missed occurrences of wxString::GetData() to GetChars( wxString ).
* Fix some minor code formatting issues.

    In netlist generation, changed the rule to calculate netnames of nets with labels:
    Previously, named nets (i.e. nets with local, hierarchical or global labels) have their name
    defined by the first label found in list.
    So net names could be changed without really changing the schematic.
    Now the names are calculated from the rules (by priority order) :
    1 - use the most top level labels in hierarchies.
    2 - use global labels first, local labels next (hidden power pins names are global labels).
    3 - use alphabetic sort (so, if GND and AGND are connected, the net will be always named AGND,
    and adding a VSS connection cannot change the net name)
    So power nets and nets that have more than one label cannot have their netname changed
    if there is no actual change relative to these nets names in schematic

    In netlist generation, changed the rule to calculate netnames of nets with labels:
    Previously, named nets (i.e. nets with local, hierarchical or global labels) have their name
    defined by the first label found in list.
    So net names could be changed without really changing the schematic.
    Now the names are calculated from the rules (by priority order) :
    1 - use the most top level labels in hierarchies.
    2 - use global labels first, local labels next (hidden power pins names are global labels).
    3 - use alphabetic sort (so, if GND and AGND are connected, the net will be always named AGND,
    and adding a VSS connection cannot change the net name)
    So power nets and nets that have more than one label cannot have their netname changed
    if there is no actual change relative to these nets names in schematic

minor enhancements. Fixing a crash in build_BOM.cpp (temporary fix, until Dick finishes fields rework)

factor some pioneering code into a the stable building block class DSNLEXER, generalize expected valued for name and value within the template field parser.

fixed bugs 593546  ( 'Update module' in module editor becomes sometimes unavailable after undo ) and 593547  ('no' confirmation in module editor overlaps modules)

fixed bugs 593546  ( 'Update module' in module editor becomes sometimes unavailable after undo ) and 593547  ('no' confirmation in module editor overlaps modules)

* Field match method expected SCH_SHEET_PATH pointer type not SCH_SHEET
  pointer type.

Added an *experimental* tool (bitmap2component) to create logos from .bmp bitmaps. Added Potrace library to convert bitmaps to polygons
    This tool uses potarce library that converts a bitmap picture (.bmp or .pgm format) to a set of polygons.
    bitmap2component converts a bitmap to a .emp footprint (that can be imported by modedit) or a .lib component that can be    imported by libedit.
    See changelog for more info

Added an experimental tool to create logos from .bmp bitmaps. Added Potrace library to convert bitmaps to polygons

Added an experimental tool to create logos from .bmp bitmaps. Added Potrace library to convert bitmaps to polygons