Doc/ README_pack.md : fleshed out 'Running' section, and included a few notes
about compiling Darkplaces for Linux.
Client qc : define EVENT_* values for the CSQC_InputEvent() callback.
Client qc : implemented parsing a "ui_describe" console command which sets
a description of a highlighted object.  Also implemented drawing it.
UI : ensure "cl_prydoncursor" is enabled, and use setcursormode(1) to ensure
that we get mouse movement events.
UI : improved UI_DrawDescription() so that the text underneath the cursor
is never clipped at the edge of the screen.
Game : properly check for current highlighted object, and when it changes
then send the new description using the "ui_describe" command.
Data / test map : added a "description" field to the monster entity.