Fix segfault on < or g keys in explore mode

When file is fully visible.
Add get_screen_string_length() function to utf8.c

It returns number of screen characters in a utf-8 encoded str.
Fix displaying of last lines in explore mode

With wrapping turned on for texts that contain characters taking
more than one character position of the screen.
Fix displaying of pane title

For paths that contain characters taking more than one character
position.
Fix displaying of bookmark menu

For paths that contain characters takin more than one character
position.
Fix displaying of status line

For file names that contain characters taking more than one character
position.
Fix a typo in assert message in utf8.c
Enhancement of the get_normal_utf8_string_widthn()

Because of wrong order of checks and assignments, a situation with
invalid return value was possible.  Now both checks are performed before
assignments.
Fix displaying of menus

For lines that contain characters taking more than one character
position.

There were some issues with determining length of a line.  And with the
position of ellipses.
Remove get_utf8_string_length() of utils/utf8.c

It was replaced by the get_screen_string_length(), which takes
characters that take more than one character position on a screen.
Don't redraw filelist for explore window on resize
Less updates for window in explore mode on resize
Fix screen update after v in explore mode
Move content of Ctrl-W X handler to other function

This way we can do aditional things in the handler.
Fix Ctrl-W H/J/K/L keys in less-like mode

When it's activated by pressing <s-tab> with active quickpreview.
Fix Ctrl-W H/J/K/L keys in normal mode

When explore mode is active in the other pane.
Fix Ctrl-W X key in normal mode

When explore mode is active in the other pane.
Less magic numbers in the modes/view.c file

Use enumeration for indexes and number of view information structures.
Add missing initializers to init_view_info()

These fields of view_info_t structure were not initialized:
 - widths
 - view
 - search_repeat

The `re` field doesn't need to be initialized.
Fix virtual lines recalculation on 'wrap' change

The guard condition added for optimization didn't take text wrapping
into account.  Now previous value of 'wrap' option is cached and checked
for a change.