[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/uart_write.c

@@ -1,7 +1,7 @@
 #include <furi.h>
 #include <string.h>
 
-void application_uart_write(void* p) {
+int32_t application_uart_write(void* p) {
     // Red led for showing progress
     GpioPin led = {.pin = GPIO_PIN_8, .port = GPIOA};
     // TODO open record
@@ -29,4 +29,6 @@ void application_uart_write(void* p) {
         // delay with overall perion of 1s
         delay(950);
     }
+
+    return 0;
 }