search functionality

This commit adds search functionality to qtermwidget. 
It adds a searchbar (initially hidden) below the terminal display. It can be shown (and hidden) by calling QTermWidget::toggleShowSearchBar
Searching is performed by a new class: HistorySearch.

A menu entry (actions->find) is added to the test application for testing (and demoing).

If this is pulled I have a commit for qterminal to use it in that app.

br. Chr.