Show a warning if using the Flatpak on Ubuntu

[AlexanderVanhee]

I made sure to only check it the first time, so users who manually fixed it can still use the app without constant warnings. This also makes it so the check isn't ran every time on all distros, saving a precious 20ms.

[kolunmi]

awesome, but could you use the libdex subprocess spawning function since you are in a fiber? example:

bazaar/src/bz-hooks.c

Lines 408 to 433 in bd1a49c

	result = dex_await (
	dex_subprocess_wait_check (subprocess),
	&local_error);
	if (!result)
	{
	g_warning ("Hook failed to exit cleanly, abandoning it now: %s", local_error->message);
	return dex_future_new_for_int (BZ_HOOK_RETURN_STATUS_CONTINUE);
	}
	stdout_pipe = g_subprocess_get_stdout_pipe (subprocess);
	stdout_bytes = g_input_stream_read_bytes (stdout_pipe, 1024, NULL, &local_error);
	if (!stdout_bytes)
	{
	g_warning ("Failed to read stdout pipe of hook, abandoning it now: %s", local_error->message);
	return dex_future_new_for_int (BZ_HOOK_RETURN_STATUS_CONTINUE);
	}
	stdout_data = g_bytes_get_data (stdout_bytes, &stdout_size);
	stdout_str = g_malloc (stdout_size + 1);
	memcpy (stdout_str, stdout_data, stdout_size);
	stdout_str[stdout_size] = '\0';
	stdout_newline = strchr (stdout_str, '\n');
	if (stdout_newline != NULL)
	*stdout_newline = '\0';

[renner0e]

CC: #583