eeshow/gui/gui.c: begin moving input (mouse, keyboard) handling to input.c (WIP)

We had input state scattered all over the place. This cleans things up.
We also merge the functions of left and (previously) middle button.

This breaks history panning.

eeshow/Makefile

@@ -14,7 +14,7 @@ NAME = eeshow
 OBJS = main.o \
        kicad/sch-parse.o kicad/sch-render.o kicad/lib-parse.o \
        kicad/lib-render.o kicad/dwg.o kicad/delta.o \
-       gui/gui.o gui/over.o gui/style.o gui/aoi.o gui/fmt-pango.o \
+       gui/gui.o gui/over.o gui/style.o gui/aoi.o gui/fmt-pango.o gui/input.o \
        file/file.o file/git-util.o file/git-file.o file/git-hist.o \
        gfx/style.o gfx/fig.o gfx/record.o gfx/cro.o gfx/diff.o gfx/gfx.o \
        gfx/text.o gfx/misc.o \