Remove unnecessary check in load_dir_list()

Which was before the call of draw_dir_list().
The check is not needed because draw_dir_list() has the same check
inside of it, but with reverse logic:

    if(curr_stats.load_stage < 2)
        return;
Extract is_dir_big() function from load_dir_list()
Extract toggle_filter_inversion() function
Fix cursor position on entering fuse mount point

When old cursor position is recalled from history and view has too
few rows.  E.g. if root of an archive contains 100 elements, the cursor
used to be on the last one and there are less then 100 rown in the view.
Fix a big similar to the one in previous commit

But this one probably can't bother anyone, so it's not added to the
ChangeLog.
Extract navigate_to() function
Extract populate_dir_list_internal() function
Add load_dir_list() function comment
Add populate_dir_list() function
Add :sync! command (with the exclamation mark)

Comparing to :sync it additionally synchronizes top line and current
cursor position of the views.
Synchronize cursor position proportionally
Fix occasional reset of cursor color to default

After pane switching operations (e.g. Ctrl-W L/H/K/J).
Forbid arguments for :sync with "!"

It doesn't make sense.
Document :sync command with "!" specified
Merge branch 'add-sync-bang'