Change some construction difficulties

The water keg and fermenting vat always felt like they should be a bit more difficult to craft, needing to be water tight. I ran a script across the construction and found the following distribution of difficulties:

    ./count_values.py difficulty type construction

    Count of values from field 'difficulty' filtered by [type=construction]
    (Data from 105 out of 7365 blobs)
    -----------------------------------------------------------------------
    0 : 60
    2 : 14
    1 : 8
    3 : 8
    6 : 4
    5 : 3
    4 : 3
    7 : 2
    8 : 2
    9 : 1

I took a stab at making the difficulty of construction a bit more fun along with "fixing" the fermenting vat and keg. I ended up trying to make the vanity stuff a bit more fun to make, and a bit more worthwhile. Examples:
* Wood fireplace and stone fireplace do pretty much the same thing. Why not make the wood fireplace a minor badge of pride at difficulty 5?
* If you have enough time to be lazy on a sofa, you've earned (difficulty 6).
* Windows are really hard to align right without proper tools. Bumped their construction difficulty to 4. (Boarding up is still at zero, gotta keep the zombies out.)

Distribution after changes:

    ./count_values.py difficulty type construction
    Count of values from field 'difficulty' filtered by [type=construction]
    (Data from 105 out of 7365 blobs)
    -----------------------------------------------------------------------
    0 : 45
    2 : 15
    1 : 10
    3 : 10
    4 : 8
    5 : 6
    6 : 6
    7 : 2
    8 : 2
    9 : 1

Everything I did by gut feel and whether I thought it would be feel more appropriate in game. I don't play with skill rust anymore, but when I did my construction skill was always pretty high, so I don't think I screwed skill rust players over.
This is likely to cause problems for new players.  Might want to try 3 or 4.