ViewFrame: call start_search() when an option is toggled

When we open the simple search, the cursor position is stored.
start_search() always search at this stored location. And when
start_search() is finished, it updates the background color for the
search state (found/not found).
ViewFrame: update up/down buttons sensitivity

The sensitivity is updated at the same time as the background color.
When the background color is red, the sensitivity is FALSE.
Regex search: do not unescape search text

\n, \t and \r are already supported by the regex search. No need to
unescape those character sequences.
Regex search: escape selection with g_regex_escape_string()
ViewFrame: do not show the number of occurrences alone

Always show it with the match position, like "3 of 10". Not "10" alone.
ReplaceDialog: use ternary conditionals
Update libgd

There is a bug fix for the tagged entry.
Search: use the new API for the regex replace errors

When a regex replace error occurs, the error icon with the tooltip is
added to the replace entry. The error icon is removed when the
replacement text changes. The new replacement text can still contain
errors, but it will be reported only when the user clicks again on the
replace button.

And when there is a replace error, the replace button is insensitive.
ReplaceDialog: don't use the regex-state property