Automatic font scaling based on screen size in the default ctwm config.

Based on a shell script which gets the DPI from the X server, and if this
fails, attempts to guess based on resolution. Taking advantage of M4 macros
in the ctwmrc, we can also scale the workspace manager and window list.

The following sizes are supported: 6x12 (<800x600) 8x16 12x24 (4k and higher)
16x32 32x64

Also makes Spleen the default font in ctwm