Make sure file from a recent menu exists before using it.

Otherwise it can result in a very confusing error that does not
suggest at all that the file is missing.

src/graphicswin.cpp

@@ -345,8 +345,13 @@ static void PopulateMenuWithPathnames(Platform::MenuRef menu,
         menuItem->SetEnabled(false);
     } else {
         for(Platform::Path pathname : pathnames) {
-            Platform::MenuItemRef menuItem = menu->AddItem(pathname.raw,
+            Platform::MenuItemRef menuItem = menu->AddItem(pathname.raw, [=]() {
-                [=]() { onTrigger(pathname); }, /*mnemonics=*/false);
+                if(FileExists(pathname)) {
+                    onTrigger(pathname);
+                } else {
+                    Error(_("File '%s' does not exist."), pathname.raw.c_str());
+                }
+            }, /*mnemonics=*/false);
         }
     }
 }

src/platform/platform.cpp

@@ -405,6 +405,13 @@ FILE *OpenFile(const Platform::Path &filename, const char *mode) {
 #endif
 }
 
+bool FileExists(const Platform::Path &filename) {
+    FILE *f = OpenFile(filename, "rb");
+    if(f == NULL) return false;
+    fclose(f);
+    return true;
+}
+
 void RemoveFile(const Platform::Path &filename) {
     ssassert(filename.raw.length() == strlen(filename.raw.c_str()),
              "Unexpected null byte in middle of a path");

src/platform/platform.h

@@ -55,6 +55,7 @@ struct PathLess {
 };
 
 // File manipulation functions.
+bool FileExists(const Platform::Path &filename);
 FILE *OpenFile(const Platform::Path &filename, const char *mode);
 bool ReadFile(const Platform::Path &filename, std::string *data);
 bool WriteFile(const Platform::Path &filename, const std::string &data);

src/solvespace.h

@@ -138,7 +138,6 @@ enum class Command : uint32_t;
 #include "platform/gui.h"
 
 const size_t MAX_RECENT = 8;
-extern Platform::Path RecentFile[MAX_RECENT];
 
 #define AUTOSAVE_EXT "slvs~"