Missed change for startskill init

Resolves #433
Default screenblocks 10, not 9

Resolves #196
setup: dynamically set size of iwad_labels array

With the addition of the Freedoom IWADs, the number of IWADs supported
by chocolate-doom has been raised to 10. However, the iwad_labels[]
array only holds place for up to 8 pointers. Incidently, I have all 10
IWADs installed and trying to warp into a game from
chocolate-doom-setup leads to an out-of-bounds access of this array
and so the application crashes with a segmentation fault.

Instead of increasing the array size to 10, which will bite us next
time, I decided to set its size dynamically as soon as the number of
IWADs of known.
opl drum note fix

fixed note drum instrument should be 60
opl voice allocation fix

opl voice allocation fix when there are no free voices
Merge pull request #467 from khokh2001/opl-fix3

Use MIDI note number 60 for percussion instruments, unless overridden
by the fixed_note field in the GENMIDI lump.
Merge pull request #468 from khokh2001/opl-fix4

Fix voice allocation when there are no more free voices. This fixes a
long-standing (but minor) discrepancy between the OPL code and the DMX 
library that was marked in the code with a TODO.
only keep gray 109 untouched if status bar numbers are from IWAD
Merge branch 'master' of https://github.com/chocolate-doom/chocolate-doom
always show numerical values next to sliders in the menu

also
- allow to disable each axis by moving the corresponding slider to 0
- shade the slider when the mouse axis is disabled
- we invert the vertical axis, not the mouse

fixes #54