setup: Fix bug opening music packs on Windows.

The previous code was opening a command prompt instead of an Explorer
window. I'm following the advice from this StackOverflow question on how
to do this properly:
<https://stackoverflow.com/questions/354902/open-in-explorer>

Thanks to /u/Ohrami3 on Reddit for discovering this bug.

src/setup/execute.c

@@ -143,8 +143,13 @@ void AddCmdLineParameter(execute_context_t *context, char *s, ...)
 
 #if defined(_WIN32)
 
-// Wait for the specified process to exit.  Returns the exit code.
+boolean OpenFolder(const char *path)
+{
+    // "If the function succeeds, it returns a value greater than 32."
+    return ShellExecute(NULL, "open", path, NULL, NULL, SW_SHOWDEFAULT) > 32;
+}
 
+// Wait for the specified process to exit.  Returns the exit code.
 static unsigned int WaitForProcessExit(HANDLE subprocess)
 {
     DWORD exit_code;
@@ -257,6 +262,22 @@ static int ExecuteCommand(const char *program, const char *arg)
 
 #else
 
+boolean OpenFolder(const char *path)
+{
+    char *cmd;
+    int result;
+
+#if defined(__MACOSX__)
+    cmd = M_StringJoin("open \"", path, "\"", NULL);
+#else
+    cmd = M_StringJoin("xdg-open \"", path, "\"", NULL);
+#endif
+    result = system(cmd);
+    free(cmd);
+
+    return result == 0;
+}
+
 // Given the specified program name, get the full path to the program,
 // assuming that it is in the same directory as this program is.
 

src/setup/execute.h

@@ -32,6 +32,7 @@ void AddCmdLineParameter(execute_context_t *context, char *s, ...) PRINTF_ATTR(2
 void PassThroughArguments(execute_context_t *context);
 int ExecuteDoom(execute_context_t *context);
 int FindInstalledIWADs(void);
+boolean OpenFolder(const char *path);
 
 txt_window_action_t *TestConfigAction(void);
 

src/setup/sound.c

@@ -22,6 +22,7 @@
 #include "m_config.h"
 #include "m_misc.h"
 
+#include "execute.h"
 #include "mode.h"
 #include "sound.h"
 
@@ -115,20 +116,7 @@ static txt_dropdown_list_t *OPLTypeSelector(void)
 
 static void OpenMusicPackDir(TXT_UNCAST_ARG(widget), TXT_UNCAST_ARG(unused))
 {
-    char *cmd;
-    int result;
-
-#if defined(__MACOSX__)
-    cmd = M_StringJoin("open \"", music_pack_path, "\"", NULL);
-#elif defined(_WIN32)
-    cmd = M_StringJoin("start \"", music_pack_path, "\"", NULL);
-#else
-    cmd = M_StringJoin("xdg-open \"", music_pack_path, "\"", NULL);
-#endif
-    result = system(cmd);
-    free(cmd);
-
-    if (result != 0)
+    if (!OpenFolder(music_pack_path))
     {
         TXT_MessageBox("Error", "Failed to open music pack directory.");
     }