overlay: update UI for card manager (#294)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Change the usage pattern for Card Manager UI.

### Previous behavior

Previously, it was a list of cards that you select, and click a button
to insert as P1 or P2.

This is fine, but users got confused when they pressed `Insert Card`
overlay key and got a different card inserted, or when auto-insert
didn't work as expected.

Additionally, if you play a game without continue (IIDX/DDR for
example), then it's cumbersome to bring up the overlay and click on
insert card every time.

### New behavior

Insert Card overlay now lets you pick a card and *slot* it into P1 or P2
side.

From that point on, the slotted card number is used as the override,
replacing what was previously passed to `-card0` or `-card1`. This means
that `Insert Card` button and auto-insert will pick up the new card and
use that instead.

## Compiling
👍 

## Testing
Tested 1p-only game (popn) and 2p games (DDR, IIDX).

src/spice2x/misc/eamuse.h

@@ -4,6 +4,7 @@
 #include <filesystem>
 #include <string>
 #include <optional>
+#include <mutex>
 
 #include "external/scard/scard.h"
 
@@ -30,6 +31,7 @@ enum eam_io_keypad_scan_code {
     EAM_IO_INSERT = 13, /* SpiceTools Extension */
 };
 
+extern std::mutex CARD_OVERRIDES_LOCK;
 extern std::string CARD_OVERRIDES[2];
 extern bool AUTO_INSERT_CARD[2];
 extern float AUTO_INSERT_CARD_COOLDOWN;