Add an option to enable QML profiling

Summary:
This adds a CMake option to enable QML profiler and debugger integration.
This allows running the gallery through the QML profiler, which makes it
simple to profile Kirigami through running Kirigami Gallery.

It is by default set to "ON" because I do not think there really is any
risk related to enabling it.

Test Plan:
With the option on, starting Kirigami Gallery from command line will print
"QML debugging is enabled". Without it, no such print happens.

Reviewers: #kirigami, bshah

Reviewed By: bshah

Subscribers: bshah, plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D25879