[DolphinWX] Prevent hotkeys from being assigned to more than one button/function.

Prior to this commit it was possible to assign the same keycode to more than one button.

ie. Say I assigned Open with the hotkey Ctrl+O; well, it was possible to also add it to another function as well, which leads to hotkey clashing.

Now, say I assign Open with Ctrl+O, but then assign that same hotkey to Refresh List; it will unbind the hotkey from Open and then assign it to refresh list.

Source/Core/DolphinWX/HotkeyDlg.cpp

@@ -72,6 +72,26 @@ void HotkeyConfigDialog::OnKeyDown(wxKeyEvent& event)
 		}
 		else
 		{
+			// Check if the hotkey combination was already applied to another button
+			// and unapply it if necessary.
+			for (wxButton* btn : m_Button_Hotkeys)
+			{
+				// We compare against this to see if we have a duplicate bind attempt.
+				wxString existingHotkey = btn->GetLabel();
+
+				wxString tentativeModKey = InputCommon::WXKeymodToString(g_Modkey);
+				wxString tentativePressedKey = InputCommon::WXKeyToString(g_Pressed);
+				wxString tentativeHotkey(tentativeModKey + tentativePressedKey);
+
+				// Found a button that already has this binding. Unbind it.
+				if (tentativeHotkey == existingHotkey)
+				{
+					SaveButtonMapping(btn->GetId(), -1, 0);
+					SetButtonText(btn->GetId(), wxString());
+				}
+			}
+
+			// Proceed to apply the binding to the selected button.
 			SetButtonText(ClickedButton->GetId(),
 					InputCommon::WXKeyToString(g_Pressed),
 					InputCommon::WXKeymodToString(g_Modkey));