Ticket #2290: make file find faster.

Gets rid of get_line_at() function that was called only at one place and
was tightly coupled with caller function. It allows to perform some
minor optimizations, and as a result improve search time by 8%.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Optimize updates of status bar during search.

Optimization is based on examining file size and time of the last status
update. This is synchronous and is based on assumption that relatively
small files are quickly processed even on slow hardware so if refresh
timeout expires during their processing, the delay couldn't be noticed
by human. Therefore only rather big files have to be reported
unconditionally, and others - only if there is a match in this file or
if timeout expires.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Refactoring of WListbox widget.

Use GQueue instead of GList to store listbox entries.

g_list_append() function is slow because it uses g_list_last()
internally to traverse from the beginning to the end of a list, so
forming a list of results has O(n*n) complexity instead of O(n).
GQueue contains pointers to head and tail of list and list length.
So in this case we don't need seach end of list every time when we
want append listbox entry to the listbox.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '2290_find_file_speedup'

* 2290_find_file_speedup:
  Refactoring of WListbox widget.
  Optimize updates of status bar during search.
  Ticket #2290: make file find faster.