firequalizer15.lua: Add a function to turn the equalizer + Smoother equalizer text, minor changes

[DaMaNic2022]

1. Add a function to turn the equalizer on and off, similar to https://github.com/mpv-player/mpv/blob/master/TOOLS/lua/acompressor.lua
2. Smoother equalizer text, minor changes:
   local freq_label = {
   eq0 = "# 0 Hz: ",
   eq1 = "# 65 Hz: ",
   eq2 = "# 157 Hz: ",
   eq3 = "# 288 Hz: ",
   eq4 = "# 472 Hz: ",
   eq5 = "# 733 Hz: ",
   eq6 = "# 1.1k Hz: ",
   eq7 = "# 1.6k Hz: ",
   eq8 = "# 2.4k Hz: ",
   eq9 = "# 3.4k Hz: ",
   eq10 = "# 4.9k Hz: ",
   eq11 = "# 7k Hz: ",
   eq12 = "# 10k Hz: ",
   eq13 = "# 14k Hz: ",
   eq14 = "# 20k Hz: "
   }
   ...
   local fontfamily = "monospace"
   ...
   local function gain_line(x)
   ...
   x = x + 10
   ...
   local function show_osd_ass()
   ...
   str = str .. pval .. freq_label[eq(x)] .. string.format("%+05.1f", gain_table[eq(x)]*0.1) .. " dB |" .. gain_line(x) .. "\n"