* Revert the changes to the screen and emulation from the last commit.

Will follow a different approach to allowing views to show different parts of the same session at once.

* Make the "Search History" menu item a normal action rather than a toggle action.
* Search terminal history and jump to first match when text in search box is changed.
* Call QApplication::processEvents() periodically while searching history to prevent UI from becoming unresponsive with large histories.