#8 Make themes be able to extend other themes

[Keith Turner]

Add the concept of themes extending other themes. This
would allow someone to create a theme that extends
another theme and changes one graphic.

For example, I could have a theme call T1 that has the
follwing theme.spec and includes one PNG called
unknown_s.png.

<<<<< begin T1's theme.spec >>>>>>

[extends]
theme=default

[planet]
unexplored=unknown_s.png

<<<<<<<<end T1's theme.spec >>>>>>

When the client reads in the T1 theme copying is not
nessecary, it points unexplored to
themes/T1/unknown_s.png and everything else points to
themes/default. So this allows a theme designer to
build upon another theme without making a copy of that
theme.

We could also make themes always extend "default" if
they do not provide an [extends] section. This way if
the user does not define something in their theme it
will automatically revert to whatever is defined in
theme "default".

We could also have multiple levels of exetension like
T2 extending T1 that extends default.