Comment 15 for bug 168658

Attached patch for basic font substitution warnings :
* Show a dialog listing which fonts have been substituted and the ability to select those affected objects
* Write warning message to the console when fonts have been substituted
* Inkscape preference (Tools->Text->Font warning dialog .. is there a better place??) to turn the warnings on/off (default is off)
* No warnings for "Sans" and "sans-serif" font substitutions - since they are assumed as "defaults"

Please test if you can, there may be some undetected cases.