Use system-agnostic return values for SaveFileYesNoCancel()

This function was returning ID{YES,NO,CANCEL}, which are specific to
Windows as return values for MessageBox(). These have been replaced with
SAVE_{YES,NO,CANCEL}, which we define ourselves.

solvespace.cpp

@@ -343,16 +343,16 @@ bool SolveSpace::OkayToStartNewFile(void) {
     if(!unsaved) return true;
 
     switch(SaveFileYesNoCancel()) {
-        case IDYES:
+        case SAVE_YES:
             return GetFilenameAndSave(false);
 
-        case IDNO:
+        case SAVE_NO:
             return true;
 
-        case IDCANCEL:
+        case SAVE_CANCEL:
             return false;
         
-        default: oops();
+        default: oops(); break;
     }
 }
 

solvespace.h

@@ -82,7 +82,11 @@ class ExprQuaternion;
 extern char RecentFile[MAX_RECENT][MAX_PATH];
 void RefreshRecentMenus(void);
 
+#define SAVE_YES     (1)
+#define SAVE_NO     (-1)
+#define SAVE_CANCEL  (0)
 int SaveFileYesNoCancel(void);
+
 // SolveSpace native file format
 #define SLVS_PATTERN "SolveSpace Models (*.slvs)\0*.slvs\0All Files (*)\0*\0\0"
 #define SLVS_EXT "slvs"

win32/w32main.cpp

@@ -873,7 +873,14 @@ int SaveFileYesNoCancel(void)
     EnableWindow(GraphicsWnd, TRUE);
     SetForegroundWindow(GraphicsWnd);
 
-    return r;
+    switch(r) {
+        case IDYES:    return SAVE_YES;
+        case IDNO:     return SAVE_NO;
+        case IDCANCEL: return SAVE_CANCEL;
+    }
+
+    oops();
+    return SAVE_CANCEL;
 }
 
 void LoadAllFontFiles(void)