Codechange: Use override specifer for overriding member declarations

This is a C++11 feature that allows the compiler to check that a virtual
member declaration overrides a base-class member with the same signature.

Also src/blitter/32bpp_anim_sse4.hpp +38 is no longer erroneously marked
as virtual despite being a template.

Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

(svn r27729) -Codechange: Do not count static NewGRF when checking for the maximum number of NewGRFs in a game.
-Codechange: Remove LAST_GRF_SLOT and MAX_NEWGRFS. Now NETWORK_MAX_GRF_COUNT is the only constant to specify the maximum number of non-static NewGRF.
-Codechange: Increase the number of file slots, effectively increasing the maximum number of static NewGRF and baseset GRFs.

(svn r23791) -Fix [FS#4960]: resize text panel for parameter description if it doesn't fit in 4 lines.
If you resize the window so it's smaller than default the text might still not fit