Added completion to defined object types to the Properties dock

This was easier than adding support for an editable combo box. Hopefully
it's just as useful in most cases. It might make sense to automatically
add suggestions for any already used type in the future.
QtPropertyBrowser: Respond to each change of text in QtLineEditFactory

I've extended this editor to allow adding a QCompleter to the line edit,
but when changing the value via the QCompleter only the 'textChanged'
and not the 'textEdited' signal is emitted.

'blockSignals' is now used to avoid this signal when applying a change
of property value, like for all other editors.
Removed remainders of the Properties dialog

All use-cases should now be fulfilled by the new Properties dock.

Closes #146
Closes #356