SearchBar: Add Cancel button to stop long running tasks

Summary:
Without this patch could it happens that Kate run an almost endless S&R
job which could lead up to an unusable system due to a run out of memory
with no possibility to cancel the job other than kill Kate, See...

CCBUG: 333517
CCBUG: 244424

Test Plan:
Before
{F6466371}

New waiting for action
{F6466378}

New while running job
{F6466380}

Reviewers: #ktexteditor, #vdg, cullmann

Reviewed By: #ktexteditor, cullmann

Subscribers: dhaumann, anthonyfieroni, brauch, cullmann, abetts, kwrite-devel, kde-frameworks-devel, #ktexteditor

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D17459