All: use double instead of int to store zoom values.

* Remove most (not all) variables that were initialized but never used that
  cause GCC 4.6 to generate a warning.
* Fix bug in schematic library editor best zoom calculation.

In Render, colors shown were the default colors, not the selected colors, when starting Pcbnew or Gerbview,
because colors were not updated after reading the config.

fix an issue created by commit 2954. Gerbview: display now an indicator of layers in use. minor code cleanup.

* Move undo item copy global variable into schematic editor frame object
  member variable.
* Add helper methods for accessing the undo item copy member variable.
* Fix undetected bug when changing a text type.
* Added an exchange command to the undo/redo base class for handling undoing
  a changed item type which cannot be undone by swapping out the variables.
* Revert change to common/hotkeys_basic.cpp that broke hot key behavior.
* Lots of coding policy changes while making the changes above.

* Gerbview, Pcbnew: add SetLocaleTo_Default() after reading double in config to fix issues with decimal separator (when the separator is not a point).
(WxWidgets 2.9.1 seems call setlocale( LC_NUMERIC, "" ) when wxConfig read doubles stored in config, but forget to back to current locale.)
Note: perhaps there are some dialogs that need this call to  SetLocaleTo_Default() ).
* Minor  code cleanup in pcbnew/netlist.cpp

* Rename EDA_Rect class to EDA_RECT.
* Rename EDA_TextStruct class to EDA_TEXT.
* Remove duplicate Doxygen comments from sch_sheet_path.cpp.

fix 2 minor bugs: GERBVIEW: Button "Erase all layers" not working, EESCHEMA: Plot to Clipboard: EDA_DRAW_FRAME::PrintPage() error

Gerbview:   Added: read Excellon files created by Pcbnew. The full Excellon command set is not supported, but drill files created by Pcbnew are supported.

* Set default button on all dialogs that do not already have a defined
  default button.  The only exception is the EESchema net list dialog
  which needs converted over to wxFormBuilder and due to it's design makes
  setting the default button in any tab other than the default tab
  problematic.
* Fix escape key dialog handling bug in EESchema print post script
  dialog.

Gerbview: tweaking code: Now the active layer is drawn on top of other layers. This is an enhancement in stacked draw modes.

others: fix minor issues.