Buff chest sizes

Make the inventory sizes of all the technic chests larger, to provide
a more appreciable benefit from upgrading chests, more in keeping with
the resources spent on the upgrades.

Currently the game engine doesn't handle large forms well.  The size
of an inventory slot is fixed relative to the screen height, and a
form that exceeds either screen dimension will extend off the screen,
making parts of it inaccessible.  The tallest a form can get and remain
usable is 13 slots, and even that slightly overspills the screen height.
The maximum usable width depends on the screen aspect ratio.  For the
narrowest common ratio, 5:4, the widest a form can usably get is 15 slots,
again slightly overspilling.  Combined with the layout of the chest forms,
this implies a maximum practical chest inventory size of 15x7 (= 105),
slightly smaller than the largest Minecraft chests.

To provide roughly equal size steps in the five upgrades from wooden
chest (staying at 8x4 = 32) to mithril chest, the steps need to be of
about 15 slots instead of the former 4.  The new sizes are:

    wooden   8x4  32
    iron     9x5  45
    copper  12x5  60
    silver  12x6  72
    gold    15x6  90
    mithril 15x7 105

To make upgrading from the old chest sizes to the new sizes more
convenient, the inventory size is now set not only upon chest construction
but also when accepting form input, at the same time as rewriting
the formspec.  So after upgrading the technic mod, viewing an existing
chest upgrades it to the new size.  The first time a pre-existing chest
is viewed its form will have the old dimensions, looking broken due
to the inventory background image now having the new number of slots.
The second time it is viewed the form will have the new dimensions,
and the full new number of slots will be usable.