Support for building and running external apps (#112)

ExternalApps/HelloWorld/main/Source/main.c

@@ -0,0 +1,29 @@
+#include <stddef.h>
+#include "TactilityC/app/App.h"
+#include "TactilityC/lvgl/Toolbar.h"
+
+/**
+ * Note: LVGL and Tactility methods need to be exposed manually from TactilityC/Source/TactilityC.cpp
+ * Only C is supported for now (C++ symbols fail to link)
+ */
+static void onShow(AppContextHandle context, lv_obj_t* parent) {
+    lv_obj_t* toolbar = tt_lvgl_toolbar_create(parent, context);
+    lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
+
+    lv_obj_t* label = lv_label_create(parent);
+    lv_label_set_text(label, "Hello, world!");
+    lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);
+}
+
+int main(int argc, char* argv[]) {
+    tt_set_app_manifest(
+        "Hello World",
+        NULL,
+        NULL,
+        NULL,
+        onShow,
+        NULL,
+        NULL
+    );
+    return 0;
+}