Support 2nd Disk][ card and improved card management (#726) (PR #741)

Support 2nd Disk][ in slot-5, via command line:
- -s5 diskii
- -s5d1 \<imagefile\>
- -s5d2 \<imagefile\>

NB. there's currently no Configuration UI support, except the Drive icons' tooltips show what's in slot-5 & slot-6 (for drive-n). So there's no way to eject the disks or insert new disks. The use-case I'm supporting it Wasteland which just has the 4 disks in the 4 drives.

Improved card management:
- Added `class Card` (in Card.h) which all other cards (that exist as classes) derive from (eg. LC,SSC,Mouse,Disk2).
- Added `class CardManager` (in CardManager.cpp\h) which now manages the 8 slots (and aux slot).
- Added `class Disk2CardManager` (in Disk2CardManager.cpp\h) which provides methods for operations that act on all Disk2 instances at the same time.
- Currently limited to just 1x SSC and 1x Mouse card (why would you need more?). This simplifies things, meaning there's no need to have dedicated SSCManager / MouseCardManager objects.
- Currently the 2nd Disk2 card can only be put into slot-5. This limitation is just due to the complexity of the Configuration UI. Having a more general drop-down per slot UI would remove this limitation.

source/SerialComms.cpp

@@ -65,10 +65,12 @@ SSC_DIPSW CSuperSerialCard::m_DIPSWDefault =
 //===========================================================================
 
 CSuperSerialCard::CSuperSerialCard() :
+	Card(CT_SSC),
+	m_uSlot(0),
 	m_aySerialPortChoices(NULL),
 	m_uTCPChoiceItemIdx(0),
-	m_uSlot(0),
-	m_bCfgSupportDCD(false)
+	m_bCfgSupportDCD(false),
+	m_pExpansionRom(NULL)
 {
 	m_ayCurrentSerialPortName.clear();
 	m_dwSerialPortItem = 0;