use size of first element in case the size of an element changes from that of a pointer in the future

pcbnew: fixed crash when opening the pad edit dialog from Preference (without current pad to edit) and closing the dialog.

Now alias changes can be undone, and are correctly updated in lib

eeschema, fixed : after a change in library list (Preference menu) the library cache was the first in priority list (now it is the last ,as it does)

All: added in popup menus hotkeys info for zoom commands

Fixed a size problem for wxSliders used in footprint properties editor (wxSlider very small and not usable). Linux specific.

Fixing vertical size of the design rules dialog to meet our minimal screen size requirement (768 pixels height)

* Changed line clipping function in gr_basic.cpp when USE_WX_ZOOM is
  enabled.
* Set wxDC clipping region to client size when drawing outside of paint
  event handler.
* Updated kicad and eeschema CMake files as the common library now
  requires linking to the polygon library.

* Corrected over zealous clipping fix in last commit.
* Tweaked clip box rectangle size.
* Fixed cross hair cursor drawing bug.