Reimplement e129755d properly.

Before this commit, quitting through menu would bring up the dialog
twice when declining to save the file.

src/platform/cocoamain.mm

@@ -1176,8 +1176,6 @@ const void *SolveSpace::LoadResource(const std::string &name, size_t *size) {
 }
 
 - (void)applicationWillTerminate:(NSNotification *)aNotification {
-    SolveSpace::SK.Clear();
-    SolveSpace::SS.Clear();
     SolveSpace::SS.Exit();
 }
 
@@ -1211,5 +1209,8 @@ int main(int argc, const char *argv[]) {
     [NSApp activateIgnoringOtherApps:YES];
     [NSApp run];
 
+    SolveSpace::SK.Clear();
+    SolveSpace::SS.Clear();
+
     return 0;
 }

src/platform/gtkmain.cpp

@@ -642,6 +642,7 @@ protected:
     }
 
     bool on_delete_event(GdkEventAny *) override {
+        if(!SS.OkayToStartNewFile()) return true;
         SS.Exit();
 
         return true;

src/solvespace.cpp

@@ -147,9 +147,6 @@ bool SolveSpaceUI::OpenFile(const std::string &filename) {
 }
 
 void SolveSpaceUI::Exit() {
-    if(!OkayToStartNewFile())
-        return;
-
     // Recent files
     for(int i = 0; i < MAX_RECENT; i++)
         CnfFreezeString(RecentFile[i], "RecentFile_" + std::to_string(i));