printers: Don't wait for activation to start search

Connect to "search-changed" signal of GtkSearchEntry instead of
"activate" signal when searching for new printers. Cancel previous
searches and start new ones as needed.

https://bugzilla.gnome.org/show_bug.cgi?id=694154
printers: Don't show tooltip on clear icon when searching for printers

The clear icon doesn't do what its tooltip says.

https://bugzilla.gnome.org/show_bug.cgi?id=694154
printers: Don't react to icon-press when searching for new printers

There is no activatable icon in the new printer dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=694154
printers: Delay searching for remote printers

Start to search for remote printers on given hostname after
500ms of user's inactivity instead of GtkSearchEntry's default
150ms (the search is very expensive operation).
Keep filtering devices list after the default 150ms and
search immediately if user activates the search entry.

https://bugzilla.gnome.org/show_bug.cgi?id=694154