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).

Eeschema: add missing rotate and mirror sheet in sheet context menu (these transforms were possible in block commands, but  not from the sheet menu).
Fix a i18n issue in field names when loading a component, if the language was changed during a session, and kicad not restarted (see Bug #1448654).
However changing the language needs restarting kicad, in order to have all messages or menus translated.

* Add code to test if any of the components in the schematic cache have been changed
  in the component libraries.
* Prompt user to accept or reject components when changes are found.
* If the user chooses to use the components in the cache, create a new library with the old
  components and add it to the beginning of the component library list so the schematic will
  not be changed.
* Create dialogs to handle user feedback and status.