Add ui.c:ui_view_is_visible() function

Checks whether view is visible on the screen.
Don't redraw invisible explore views

Thanks to filterfalse.
Update ui.c:switch_views() function

 * Rename it to swap_view_roles().
 * Add leading comment.
 * Move after its last invocation.
Simplify ui.c:change_window() function

Remove a bunch of old code with a better single line.
Fixes issue with blank view on switching to invisible view in
explore mode.
Move quickview/explore title composing

From quickview.c:quick_view_file() to filelist.c:update_view_title().
Allow disabling hidden quickview with :view
Add ui.c:ui_views_update_titles()

Updates title of the views.
Use ui_views_update_titles() function
Fix update of titles with viewed-quickview

:view and then <c-w>w
Document UI fixes
Move update_view_title() from filelist.c to ui.c
Rename update_view_title -> ui_view_title_update
Merge branch 'fix-single-view-explore-mode-redraw'

Allow disabling quickview while its pane is hidden.

Fix drawing of hidden view in explore mode over active one.  Thanks to
filterfalse.

Fix missing title of quickview pane after entering view mode (e.g. with
Ctrl-W W).

Fix titles update when one of panes is in view mode (active and inactive
highlighting wasn't always right).