fbt: changed stock plugins to EXTERNAL apps; restored building app as a PLUGIN as a part of main fw as well as a .fap; readme fixes

applications/plugins/bt_hid_app/application.fam

@@ -1,7 +1,7 @@
 App(
     appid="bt_hid",
     name="Bluetooth Remote",
-    apptype=FlipperAppType.PLUGIN,
+    apptype=FlipperAppType.EXTERNAL,
     entry_point="bt_hid_app",
     stack_size=1 * 1024,
     cdefines=["APP_BLE_HID"],

applications/plugins/music_player/application.fam

@@ -1,7 +1,7 @@
 App(
     appid="music_player",
     name="Music Player",
-    apptype=FlipperAppType.PLUGIN,
+    apptype=FlipperAppType.EXTERNAL,
     entry_point="music_player_app",
     cdefines=["APP_MUSIC_PLAYER"],
     requires=[

applications/plugins/picopass/application.fam

@@ -1,7 +1,7 @@
 App(
     appid="picopass",
     name="PicoPass Reader",
-    apptype=FlipperAppType.PLUGIN,
+    apptype=FlipperAppType.EXTERNAL,
     entry_point="picopass_app",
     requires=[
         "storage",

applications/plugins/snake_game/application.fam

@@ -1,7 +1,7 @@
 App(
     appid="snake_game",
     name="Snake Game",
-    apptype=FlipperAppType.PLUGIN,
+    apptype=FlipperAppType.EXTERNAL,
     entry_point="snake_game_app",
     cdefines=["APP_SNAKE_GAME"],
     requires=["gui"],