Add simple preference window for modifying system audio settings:

* Still need to add actual behaviour!
Start adding implementation for Qt GUI system audio preferences:

* Still much more polish to do
* Volume slider appears to be logarithmic rather than linear
* Radio buttons don't persist value
* Volume slider isn't initialised to system value
* Radio buttons aren't initialised to system value
* Thanks to Zack Igielman for suggesting the specific amixer commands.
add missing header changes for previous commit
add missing include
remove unnecessary references to toolbox
Handle the fact that the mixer percentage range isn't linear:

* Also try updating system vol *whilst* sliding
Pull in cmath on RPi and namespace pow correctly
just initialise preferences window once rather than re-creating on view
pull in correct qtconcurrent headers for system
add missing ostringstream line
update splash image to remove whitespace surround
give splash image more vertical space
Add missing ostringstream declaration for OS X Qt GUI
Stop comments going invisible if they're the current line
Add more appropriate comment for new buffers :-)
rename techno fx to ixi_techno in synthinfo
Use basic HTML formatting on error pane to improve readability.
Dock prefs pane in right dock by default - also add new prefs:

* buttons to show output and error panes (in case the user closed them)
* tick boxes to turn off output and synth arg checking for new jobs (not 
  implemented yet)
remove cruft
Don't raise an error window if user cancels buffer  save