various: implement sequence pixel aspect ratios and interlacing settings

Implements the following:
- Sequences have pixel aspect ratios that work in tandem with footage PARs
  to render footage correctly. Viewer and export also acknowledge PARs
- Sequences can have interlacing settings. This doesn't do anything yet,
  eventually the renderer will need to interlace/deinterlace/reinterlace
  appropriately in order to conform all the footage to the sequence. Export
  acknowledges interlacing, but this only affects metadata, not the image.

app/common/ratiodialog.cpp

@@ -70,7 +70,8 @@ double GetFloatRatioFromUser(QWidget* parent,
 
       if (numer_ok
           && denom_ok
-          && num > 0) {
+          && num > 0
+          && den > 0) {
         // Exit loop and set this ratio
         if (ok_in) {
           *ok_in = true;