trunk: hiding a layer or object fails unless element already has 'display' style property

[su_v]

In recent trunk, hiding a layer (via Layers dialog or the layer controls in the status bar), or hiding an object via Object Properties dialog, fails unless the element already has a 'display' property defined in the 'style' attribute.

Steps to reproduce:
1) launch trunk (default prefs, default new doc)
2) draw a rect
3) hide current layer

Expected result:
The just drawn rect disappears from canvas because the layer visibility has been turned off.

Actual result:
The layer toggle for visibility has no effect.

Notes:
* Custom templates with layers might not expose the reported regression if the visibility mode of existing layers had been modified before the SVG file was saved as template.
* Layers in existing documents which have had the visibility changed in past sessions with older trunk builds (or stable versions) are not affected.
* Not reproduced with lp:inkscape r15470 - possibly, this regression is related to the merge of the style dialog in r15471.
* Other layer (or object) properties (lock, layer blend modes) seem not affected.

Known workaround:
Add 'display:inline' to the element's 'style' attribute in the XML Editor.

Reproduced with lp:inkscape r15485 on Ubuntu 14.04.5 LTS.

[jazzynico (jazzynico)]

Confirmed on Xubuntu 16.04, lp:inkscape rev. 15484.

[Tavmjong Bah (tavmjong-free)]

The Layer dialog calls SPItem::setHidden() which sets the 'display' property directly without setting the style_src variable which is initialized to SP_STYLE_SRC_UNSET. As writing properties directly is likely to exist throughout Inkscape's code base, it is probably safer to initialize style_src to SP_STYLE_SRC_PROP.

[su_v (suv-lp)]

Fix committed by Tavmjong Bah in rev 15490:
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/15490
confirmed with lp:inkscape r15492 on Ubuntu 14.04.5 LTS.