task_database: use regular progress report

[natinusala]

This commit makes it so the database task uses regular progress report instead of calling runloop_msg_queue_push manually.

The progress is now shown as a percentage instead of number scanned / total number of files. Only the file name of each scanned file is displayed instead of the full path (to reduce the width of the onscreen message).

The alternative_look flag was added to tasks, set it to true to notify that the task title will change during its execution. It prevents the task menu widget from being illegible due to the title change animation.

This has been tested on PC with both menu widgets enabled and disabled, as well as the Qt interface.

[inactive123]

This crashes after a scan is over.

Thread 7 received signal SIGSEGV, Segmentation fault.
[Switching to Thread 9840.0x2148]
0x00007fff55fad826 in strrchr () from C:\WINDOWS\System32\msvcrt.dll
(gdb) bt
#0 0x00007fff55fad826 in strrchr () from C:\WINDOWS\System32\msvcrt.dll
#1 0x0000000000433c93 in path_basename ()
#2 0x000000000058a31b in task_database_iterate_start ()
#3 0x000000000058b2ae in task_database_handler ()
#4 0x000000000041eaa9 in threaded_worker ()
#5 0x0000000000680df3 in thread_wrap ()
#6 0x00007fff56004034 in KERNEL32!BaseThreadInitThunk ()
from C:\WINDOWS\System32\kernel32.dll
#7 0x00007fff58b03691 in ntdll!RtlUserThreadStart ()
from C:\WINDOWS\SYSTEM32\ntdll.dll
#8 0x0000000000000000 in ?? ()

I'll revert this PR for now. You can send me a better version of this PR, maybe try also testing it through ASAN/TSAN.