Update default depth to 5 + less hardcoding in documentation

[inariksit]

As requested by Michal:

Define in CommonCommands

default_depth :: Int
default_depth = 5

and use that value in documentation + giving default values to GF shell commands. Typing help gr in the GF shell gives the following output:

Lang> help gr
gr, generate_random
generate random trees in the current abstract syntax

syntax:
  gr [-cat=CAT] [-number=INT]

Generates a list of random trees, by default one tree up to depth 5.
If a tree argument is given, the command completes the Tree with values to
all metavariables in the tree. The generation can be biased by probabilities,
given in a file in the -probs flag.


flags:
 -cat   generation category
 -lang  uses only functions that have linearizations in all these languages
 -number        number of trees generated
 -depth the maximum generation depth (default: 5)
 -probs file with biased probabilities (format 'f 0.4' one by line)

examples:
  gr    --one tree in the startcat of the current grammar, up to depth 5
  gr -cat=NP -number=16 --16 trees in the category NP
  gr -cat=NP -depth=2   --one tree in the category NP, up to depth 2
  gr -lang=LangHin,LangTha -cat=Cl      --Cl, both in LangHin and LangTha
  gr -probs=FILE        --generate with bias
  gr (AdjCN ? (UseN ?)) --generate trees of form (AdjCN ? (UseN ?))

I did not change the default depth for server from 4, but I put it in a variable.