Prevent image preview flicker for load command

[joelim-work]

• Fixes custom on-select triggers file preview twice #1333

This is a continuation from the discussion in #1281. In my opinion the load command should not refresh the image preview, but only check whether files/directories have been modified. In the case that there are modifications, the image preview will be refreshed through calling the nav.checkDir and nav.checkReg functions. This is the same as how the period timer behaves, and is also implied by the documentation:

lf/doc.go

Lines 823 to 829 in 2f3c170

	period int (default 0)
	Set the interval in seconds for periodic checks of directory updates.
	This works by periodically calling the 'load' command.
	Note that directories are already updated automatically in many cases.
	This option can be useful when there is an external process changing the displayed directory and you are not doing anything in lf.
	Periodic checks are disabled when the value of this option is set to zero.

I understand the load command is used internally after updates such as rename, delete and paste, but in each case there will be modifications to the filesystem so the image preview will be refreshed anyway as described above. For me this works fine, but I suppose it would be nice if others test it as well, preferably with image previews.

[tex]

Tested it, works like a charm!