QmlJsTools: Move category_qml from Core to QmlJsTools

It is qml specific, and only used in qml plugins.

Alsoe, SETTINGS_CATEGORY_QML_ICON was defined in
qmldesignerconstants.h and other plugins included that
header. This contradicts the plugin dependecies
too much.

Change-Id: I76f31cd01bc9c02faf914051cae8bac7d7bac659
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>