Improved Touch support for Kickoff

Summary:
- enabled touch scrolling in Kickoff
- open the correct category in the application tab
- open the context menu with touch (tapandhold gesture similar to Windows 10)
- start drag action with touch similar to Windows 10

BUG: 406359
BUG: 406361
FIXED-IN: 5.18

Reviewers: #plasma, hein, ngraham

Reviewed By: ngraham

Subscribers: niccolove, khuoman, mart, sbergeron, davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21829