introduce nodebox abstractions used by fences, like corner nodeboxes, and indirectly slab_x; update fences accordingly

A quick edit to drop items from an itemframe or pedestal when dug instead of removing them, also a fix for the abm to update pedestals properly...

should fix this issue:
https://forum.minetest.net/viewtopic.php?f=11&t=2041&start=625#p185494

add some informative messages

(default gold + steel + copper == 3 brass)

- Rename extra font dirs to match sizes (were off by a few px)
- Distribute colors per font, and resize default color textures
  to match default font.
- Removed character size cache (was useless and just made things
  more complex).
- Get line height from background color texture height.

as used by CWz's mod of the same name

Write @KEYWORD (all caps, with the 'at') into the sign's text to display
the keyword.  Keyword displayed is automatically updated if the 15-second
update interval detects that it differs from the minetest.conf setting.

The keyword setting is only read at startup.

delete an obsolete texture.