Add a temporary name filter for the main view.

By using 'F' in the main view you can enter a regex
that will be used to filter the view based on the torrent name.
To remove the filter set the input to empty.
This is adding a 'view.temp_filter' property and a 'match{}' function.
For example with Ctrl+X you could do:
command> view.temp_filter=main,"match={d.name=,.*linux.*iso}"

src/core/view_manager.cc

@@ -109,6 +109,14 @@ ViewManager::set_filter(const std::string& name, const torrent::Object& cmd) {
   (*viewItr)->filter();
 }
 
+void
+ViewManager::set_temp_filter(const std::string& name, const torrent::Object& cmd) {
+  iterator viewItr = find_throw(name);
+
+  (*viewItr)->set_temp_filter(cmd);
+  (*viewItr)->filter();
+}
+
 void
 ViewManager::set_filter_on(const std::string& name, const filter_args& args) {
   iterator viewItr = find_throw(name);