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.