Clear konsole command-line before directory change

Summary:
When changing tabs anything that exists in the command line gets
executed.
That can be very dangerous. Dolphin clears the command line. I
copied/pasted that code.

Test Plan:
Open 2 files residing in different directories in kate
Activate konsole plugin.
In konsole type 'ls /tmp' (not ENTER)
Change tab.

-> ls /tmp should not execute

Differential Revision: https://phabricator.kde.org/D9590