* auto-save was broken for all sub sheets with are in a sub-directory of the project directory. (reason wxFileName::Normalize(), used to create the absolute path  has issues with filenames starting by '$', perhaps due to env var which also start by $). the auto-save prefix is modified ( now AUTOSAVE_PREFIX_FILENAME which defines "_save_"  ) and defined only once.
* auto-save files from sub-sheets were not found due to the fact the path was not set (fixed by Blair Bonnet's patch)

Fix 2 coverity warnings.

tools/edit_tool.cpp: remove limitation (GAL mode) to delete the last pad in a footprint (a footprint can have 0 pad, for instance a logo)

*  Use only double to store zoom levels in class BASE_SCREEN (int type is not working for eeschema or page layout editor) (this fix Bug #1450551, Assertion failed on wxRound() when zooming)
*  Fix crash in all apps but Pcbnew when a grid or a zoom was selected from the popup menu.
*  Modify default hotkeys for Next Grid and Previous Grid: the initial key was a dead key, not suitable as hotkey. the 'N' and Shift N keys are used instead (with a change for unfill zone hotkey)
*  Modify code in EDA_DRAW_FRAME::SetNextGrid() and EDA_DRAW_FRAME::SetPreviousGrid() to make them compatible with other editors than the board editor.

added the example gen_gerber_and_drill_files_board.py in demos, which shows how to do that.
Fix a Printf format issue (shown in Debug mode) in Libedit (%d used for a size_t, changed in %zu)

Minor fixes: dialog rescue: make strings translatable. Remove a few coverity warnings (not initialized members). Very minor other changes.

Add a python script example in demo folder to create various plot files from a board. Fix also some issues in plot controller helper class.
Important note: from a python script one cannot plot the palge layout, because the page layout template file is not stored in the board file, but it is part of the project.
Because when using a python script, the project is not loaded, the page layout template is not known.
Trying to plot it crashes the script.

Minor fixes in dialog rescue: missing title, incorrect refresh due to use of a wxPaintDC outside a paint event, and coverity warnings (not initialized members).