Resolve "SDL2 module"

Closes #29 (closed)

• create a working sdl module
• using only async commands with mpv
• put the sdl module in its own thread
• make the thing working

inc/lektor/module/module_sdl2.h

+#pragma once
+
+#include <lektor/defines.h>
+#include <stdbool.h>
+#include <sqlite3.h>
+#include <lektor/window.h>
+
+/* The only function with a setted filename */
+int module_set_function(void *mod, void *handle);
+
+/* Below there are needed functions for a window module.
+ * Names are not fixed but they follow a convention (to get a code easy
+ * to read). They should not be used in lektor! They will be loaded into
+ * the struct lkt_win by the module_set_function.
+ */
+bool module_sdl2_new(struct lkt_win *const win);
+void module_sdl2_close(struct lkt_win *const win);
+void module_sdl2_free(struct lkt_win *const win);
+
+bool module_sdl2_toggle_pause(struct lkt_win *const win);
+bool module_sdl2_load_file(struct lkt_win *const win, const char *filepath);
+bool module_sdl2_set_volume(struct lkt_win *const win, int vol);
+
+bool module_sdl2_get_duration(struct lkt_win *const win, int *dur_sec);
+bool module_sdl2_get_elapsed(struct lkt_win *const win, int *elapsed_sec);
+
+bool module_sdl2_handle_events(struct lkt_win *const win, sqlite3 *db, enum mpd_idle_flag *mpd_idle_events);