UI: improve open/save NC dialogs
UI: improve open/save NC dialogs

"Open NC" dialog:
 * default directory is set to project directory (or empty if project does not have filename)
 * add a filter on extension, but add a selectable "all files" filter too

" Save NC" dialog:
 * default directory is set to project directory (or "DocumentsDir" if project does not have filename)
 * default filename is based on GetOutputFileName()
 * add a filter on extension, but add a selectable "all files" filter too


singwina/heekscnc,elliotf/heekscnc,dnevels/heekscnc,elliotf/heekscnc,blakelyc/heekscnc,BbiKkuMi/heekscnc,ant-t/heekscnc,AlanZheng/heekscnc,singwina/heekscnc,Nurb432/heekscnc,Nurb432/heekscnc,JamieFBousfield/heekscnc,hackendless/heekscnc,sangit/heekscnc,blakelyc/heekscnc,dnevels/heekscnc,hackendless/heekscnc,Nurb432/heekscnc,BbiKkuMi/heekscnc,hackendless/heekscnc,masika/heekscnc,elliotf/heekscnc,FluffyMortain/heekscnc,FluffyMortain/heekscnc,AlanZheng/heekscnc,BbiKkuMi/heekscnc,singwina/heekscnc,dnevels/heekscnc,AlanZheng/heekscnc,hackendless/heekscnc,pyrotron/heekscnc,ant-t/heekscnc,pyrotron/heekscnc,FluffyMortain/heekscnc,ant-t/heekscnc,dnevels/heekscnc,FluffyMortain/heekscnc,sangit/heekscnc,sangit/heekscnc,elliotf/heekscnc,blakelyc/heekscnc,singwina/heekscnc,pyrotron/heekscnc,ant-t/heekscnc,blakelyc/heekscnc,sangit/heekscnc,pyrotron/heekscnc,masika/heekscnc,masika/heekscnc,AlanZheng/heekscnc,JamieFBousfield/heekscnc,JamieFBousfield/heekscnc,Nurb432/heekscnc,JamieFBousfield/heekscnc,masika/heekscnc,BbiKkuMi/heekscnc