convert lvgl from submodule to a plain old directory

lib/lvgl/docs/porting/timer-handler.md

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+# Timer Handler
+
+To handle the tasks of LVGL you need to call `lv_timer_handler()` periodically in one of the following:
+- *while(1)* of *main()* function
+- timer interrupt periodically (lower priority than `lv_tick_inc()`)
+- an OS task periodically
+
+The timing is not critical but it should be about 5 milliseconds to keep the system responsive.
+
+Example:
+```c
+while(1) {
+  lv_timer_handler();
+  my_delay_ms(5);
+}
+```
+
+If you want to use `lv_timer_handler()` in a super-loop, a helper function`lv_timer_handler_run_in_period()` is provided to simplify the porting:
+
+```c
+while(1) {
+    ...
+    lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
+    ...
+}
+```
+
+ In an OS environment, you can use it together with the **delay** or **sleep** provided by OS to release CPU whenever possible:
+
+```c
+while (1) {
+    lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
+    my_delay_ms(5);                    /* delay 5ms to avoid unnecessary polling */
+}
+```
+
+To learn more about timers visit the [Timer](/overview/timer) section.
+