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/overlay/windows/card_manager.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <filesystem>
+#include <optional>
 
 #include "overlay/window.h"
 
@@ -10,6 +11,7 @@ namespace overlay::windows {
         std::string name = "unnamed";
         std::string id = "E004010000000000";
         std::string search_string = "";
+        bool read_only = false;
         float color[3] {};
     };
 
@@ -29,7 +31,10 @@ namespace overlay::windows {
         char name_buffer[65] {};
         char card_buffer[17] {};
         float color_buffer[3] {};
+        
+        std::optional<CardEntry> card_cmd_overrides[2];
 
+        std::optional<CardEntry> loaded_card[2];
         CardEntry *current_card = nullptr;
 
         std::string search_filter = "";
@@ -41,10 +46,13 @@ namespace overlay::windows {
         void generate_search_string(CardEntry *card);
         void generate_random_color();
 
-        void build_card();
+        bool build_card(int reader);
         void open_card_editor();
         void build_card_editor();
         void build_card_list();
+        void build_card_selectable(CardEntry &card);
         void build_footer();
+
+        void insert_card_over_api(int reader, CardEntry &card);
     };
 }