Add utils/filter.c unit

It provides a small abstraction over filter driven by a regular
expression.
Fix clearing filter by setting it to empty string
Provide a separate filter for "folds"

For files that are hidden using zf keybinding.
Note mapping commands in quick view section of doc
Store auto file filter to vifminfo
Rename filename filters check function

regexp_filter_match => file_is_visible

Also correct heading comment of the function.
Use filter_t for filename filter
Remove set_filename_filter() function

Replace it with calls to functions of the utils/filter module.
Rename prepare_view{,s}() functions

prepare_view => reset_view
prepare_views => reset_views

It reflects what they do better.
Display state of both filters on :filter?
Mirror name filter operations on auto filter
Fix filter resetting
Update documentation on filters
Fix filtering only files that match filter

Filtering files out used did work.
Better highlighting of strings

Fix highlighting for :cd and :source commands

Thanks to filterfalse.
Merge branch 'separate-filename-filters'