QtPropertyBrowser: Allow controlling whether to show text for bool values

This was a rather involved patch, but it avoids the need for using
private API or duplicating QtBoolEdit (which is in a private header).

Apart from adding support for this QtBoolEdit option to the
QtCheckBoxFactory and the QtBoolPropertyManager, the setting can also
be controlled via the 'textVisible' attribute associated with
QtVariantProperty instances.
Use the new attribute to hide the text of boolean properties
QtPropertyBrowser: Document the 'echoMode' attribute for string values
Moved up the 'Visible' attribute for objects

More closely matches the location of this attribute for layers.
Allow changing the image layer image in the Properties dock

Introduced a 'filePath' property with a special editor that features
a small button that opens up the file chooser. It also colors the text
to make it clear whether the file path exists or not.

The VariantPropertyManager, VariantEditorFactory and FileEdit classes
were based on the example from issue 18 of the Qt Quarterly [1]

[1] http://doc.qt.digia.com/qq/qq18-propertybrowser.html
Removed layer property dialogs

All layer properties, including those of object layers and image layers,
can now be edited in the new Properties dock.