Include SDL as subproject to fix CI simulator build (#158)

Documentation/ideas.md

@@ -13,14 +13,9 @@
 - EventFlag: Fix return value of set/get/wait (the errors are weirdly mixed in)
 - Consistently use either ESP_TARGET or ESP_PLATFORM
 - tt_check() failure during app argument bundle nullptr check seems to trigger SIGSEGV
-- Fix bug in T-Deck/etc: esp_lvgl_port settings has a large stack depth (~9kB) to fix an issue where the T-Deck would get a stackoverflow. This sometimes happens when WiFi is auto-enabled and you open the app while it is still connecting.
+- Fix bug in T-Deck/etc: esp_lvgl_port settings has a large stack size (~9kB) to fix an issue where the T-Deck would get a stackoverflow. This sometimes happens when WiFi is auto-enabled and you open the app while it is still connecting.
 
 # TODOs
-- Try to fix SDL pipeline issue with apt-get:
-  ```yaml
-  - name: Install SDL
-    run: sudo apt-get install -y libsdl2-dev cmake
-  ```
 - Make "blocking" argument the last one, and put it default to false (or remove it entirely?): void startApp(const std::string& id, bool blocking, std::shared_ptr<const Bundle> parameters) {
 - Boot hooks instead of a single boot method in config. Define different boot phases/levels in enum.
 - Add toggle to Display app for sysmon overlay: https://docs.lvgl.io/master/API/others/sysmon/index.html