[FL-872] Furi, API-HAL, App-Loader cleanup and improvements (#334)

* Furi: replace obsolete furiac_exit with osThreadExit, drop obsolete apis and test. Rename systemd to flipper and move to separate file, cleanup. ApiHal: Rename timebase to os and move freertos hooks there, move insomnia api to power module.
* Furi: new thread helper
* Furi: cleanup thread documentation
* Flipper, AppLoader: update to use FuriThread. Update tasks signatures to match FuriThreadCallback signature.
* F4: rename API_HAL_TIMEBASE_DEBUG to API_HAL_OS_DEBUG
* Applications: rename FuriApplication to FlipperApplication, use FuriThreadCallback signature for apps.
* C++ app template sample, new exit method

applications/examples/u8g2_example.c

@@ -1,7 +1,7 @@
 #include "u8g2/u8g2.h"
 #include <furi.h>
 
-void u8g2_example(void* p) {
+int32_t u8g2_example(void* p) {
     // open record
     u8g2_t* fb = furi_record_open("u8g2_fb");
     u8g2_SetFont(fb, u8g2_font_6x10_mf);
@@ -10,5 +10,5 @@ void u8g2_example(void* p) {
     u8g2_DrawStr(fb, 2, 12, "hello world!");
     furi_record_close("u8g2_fb");
 
-    furiac_exit(NULL);
+    return 0;
 }