Prevent cursor jumping to bottom after deleting file

[joelim-work]

Fixes #1236

This happens because by default the dir.sel function tries to move the cursor to the bottom of the window, which is supposedly a reasonable default if the current cursor position isn't known.

Anyway I changed the code so that when a directory changes (e.g. a file gets deleted), the cursor position for the updated directory will be copied from the directory cache entry, and then dir.sel will just simply apply 'bounds checking' to ensure the cursor doesn't end in an invalid location.

It turns out this kind of check is reusable, so I extracted it out into a separate function and applied it in the nav.up and nav.down commands as well.

I have tested these changes a fair bit myself, but I would appreciate it if someone else also tested it too, in case I missed something.

[gokcehan]

@joelim-work It looks good, thanks for the patch.

[ilyagr]

FYI, I noticed this changes the behavior of lf if it shows at least 3 columns and you do the following:

$ mkdir t{1..100}                                                     
$ cd t55
$ lf

Previously, as lf starts, the parent directory in the column one left from the active cursor was shown at the bottom of the list. Now, it's at the top.

I don't think this is a problem, it could even be considered a minor improvement. My ultimate preference would be for the parent directory to be in the middle, but I don't think that's important.