project: implement searching

I can't believe this has been unimplemented this whole time...

app/widget/projectexplorer/projectexplorer.cpp

@@ -67,6 +67,7 @@ ProjectExplorer::ProjectExplorer(QWidget *parent) :
 
   // Set up sort filter proxy model
   sort_model_.setSourceModel(&model_);
+  sort_model_.setFilterCaseSensitivity(Qt::CaseInsensitive);
   sort_model_.setSortRole(ProjectViewModel::kInnerTextRole);
 
   // Add tree view to stacked widget
@@ -301,6 +302,11 @@ void ProjectExplorer::RenameSelectedItem()
   }
 }
 
+void ProjectExplorer::SetSearchFilter(const QString &s)
+{
+  sort_model_.setFilterFixedString(s);
+}
+
 void ProjectExplorer::ShowContextMenu()
 {
   Menu menu;