Updated Linux binaries to work with different libpng versions (helps #89).

The compiled binary always has version of libpng as 1.2.0 (even though
wxwidgets may come with a different version) and is compiled with libpng
set to use system library. The warnings from libpng about different
versions are suppressed when bitmaps are loaded. This has been tested to
work with libpng 1.2.x and 1.5.x.

This update also adds a workaround for "lsb_release: command not found"
message reported on some Linux systems.
Updated linux binaries to fix libpng support (helps #89).

Binaries have been updated to use whatever system libpng library is
available (without being linked against a particular version).

This also includes fix for 64bit systems to allow for these binaries to
work with "older" versions of libpng (for example, v1.2).

The binaries include wxwidgets fix for lsb_release messages, so the
workaround for this issue has been removed.
Fixed function list in toolbar on Linux (helps #89).
Improved performance of tab switching and project tree population.
Added check for different spellings of the same folder in the project tree.
Updated Windows/OSX build files to only build components needed (helps #89).
Added scripts to install build prerequisites on Linux (helps #89).
Added custom configuration file cfg/user.lua