All: fixing a recent problem when using scroll bars: the full screen was redrawn, that can be very slow in Pcbnew.

* Make USE_WX_ZOOM clipping routine actually clip rather than just test
  if line needs drawn.
* Clean up as many USE_WX_ZOOM #ifdefs as possible.
* Minor coordinate rounding improvements.
* Minor scrolling and panning improvements.
* Remove unused KicadGraphicContext object code.

    1)  nested quotes.  This is in anticipation of broader usage of the
        file type/syntax.  A string like this in the file:
            "my ""favorate"" string"
        can be returned as
            my "favorite" string
    2)  CommentsAsTokens is implemented, so you can ask the lexer to return
        comments as tokens, so they can be preserved.  The default is to ignore
        them.  A comment is defined as any line that has # as its first
        non-blank character.  (This means comments cannot follow anything else
        on a line.)

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