Disable while fullscreen app is focused (#29)

Fixes Issue #27 

Disables hot corner while a focused app is fullscreen. I have verified that it works in the following cases:
- Exclusive fullscreen games
- Borderless fullscreen games
- Fullscreen apps entered with F11/Alt+Enter (such as Firefox)
- Fullscreen videos (YouTube)
- LibreOffice Impress while in presentation

At only 10 additional lines of code, I believe this will make the program more useful for the majority of users, while maintaining the simplicity of the program. This also brings the program closer to proper Gnome hot corner functionality.

Anyone that does not desire this functionality need only remove these additional lines before compilation.

hotcorner.c

@@ -1,6 +1,7 @@
 #define WIN32_LEAN_AND_MEAN
 #include <stdlib.h>
 #include <windows.h>
+#include <shellapi.h>
 
 #pragma comment(lib, "USER32")
 #pragma comment(linker, "/SUBSYSTEM:WINDOWS")
@@ -90,6 +91,7 @@ static DWORD WINAPI CornerHotFunc(LPVOID lpParameter)
 static LRESULT CALLBACK MouseHookCallback(int nCode, WPARAM wParam, LPARAM lParam)
 {
     MSLLHOOKSTRUCT *evt = (MSLLHOOKSTRUCT *) lParam;
+    QUERY_USER_NOTIFICATION_STATE pquns;
 
     // If the mouse hasn't moved, we're done.
     if (wParam != WM_MOUSEMOVE)
@@ -123,6 +125,17 @@ static LRESULT CALLBACK MouseHookCallback(int nCode, WPARAM wParam, LPARAM lPara
         goto finish;
     }
 
+    // Check if an app is currently fullscreen.
+    SHQueryUserNotificationState(&pquns);
+
+    switch (pquns) {
+    case QUNS_BUSY:
+    case QUNS_RUNNING_D3D_FULL_SCREEN:
+    case QUNS_PRESENTATION_MODE:
+        goto finish;
+    default:
+    }
+
     // The corner is hot, and was previously cold. Here we start a thread to
     // monitor if the mouse lingers.
     CornerThread = CreateThread(NULL, 0, CornerHotFunc, NULL, 0, NULL);