Fixes to Datainput metadata API

Runtime API fixes:

• C++: Q3DSPresentation::dataInputs() and dataInputs(QVariant &metadatakey), both returning QVector<DataInput*>. QVariant Q3DSDataInput::metadata(QVariant &key) and Q3DSDataInput::setMetadata(QVariant &key, QVariant &value). QVariantList Q3DSDataInput::metadataKeys().
• QML: getDataInputs() that is a wrapper to datainputs(), returns QVariantList. Wrappers that match C++ setMetadata() and metadata(key) (all return / inputs are QVariants). QML wrapper getMetadataKeys() for C++ metadataKeys(), returns QVariantList. 
• Runtime internal : change metadata entry to use QHashMap i.e. allow multiple key-value pairs per datainput.

Editor:

• change DI dialog to allow entering multiple metadata key-value pairs

Common:

• writing and parsing of UIA files to support full metadata maps instead of single tags

NOTE: On QML side, metadata property changes are only emitted for changes made through this datainput object. If metadata is changed elsewhere, property change will not be emitted (but current value will be available by calling getter explicitly). This behavior matches all other datainput properties which are also managed centrally by Q3DSEngine (value, min/max...).