Relabel "Mouse Click Emulation" to "Tap to Click"

Summary:
Text change/usability improvement. Replace technical terms with user-friendly language.

BUG: 398938
FIXED-IN: 5.15.0

Test Plan:

• Recompile/reinclude touchpad KCM
• Check for text change

Reviewers: ngraham, Plasma

Reviewed By: ngraham

Subscribers: romangg, furkantokac, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15699