Initialize members of AnnotatedScenePreview.

CID 12494
Initialize m_indexType.

CID 12493
SGWireFrameWidget: check for valid index type.

CID 12478
Initialize prevIndexN with some magic value.

The initial values of prevIndex1, prevIndex2 and prevIndex3 are
totally unused, they are always initialized before actually being
used. Though coverity complains about it, because e.g. prevIndex2
*is* used uninitlizedly e.g. for setting prevIndex3 (which isn't used
before getting a proper value either).
This commit is basically to steady coverity, still it assigns some
magic value, that could halp to detect an hypothetical error.

CID 12487
Replace icons visualize* tools with dedicated ones

In the QuickInspector the scene preview panel has a toolbar with tools
to enable some QSG features, visualizing stuff. These toolbuttons had
some badly fitting icons from oxygen for the beginning. This commit
replaces them by dedicated icons, properly designed by Nuno.
Add description for the visualize* tools' tooltips
Fix deselect of tool in quick scene preview.

When clicking a selected tool in the toolbar of the quick inspector's
scene preview, the render mode would be deactivated and the button's
state would be reset to checked=false, but the ExclusiveButtonGroup
would still have that button set as the current, causing it not to be
able to be reactivated without activating another tool before. Now the
current button is reset to null, causing the button to be unchecked
and the internal state to be correct.
Update margins correctly on size change.

The quick scene preview image has a margin of the size of the preview
panel, in order to scroll half over the image size (in order to have
e.g. the right end of the scene displayed in the center of the panel).
This margin was not correctly updated when the panel size changed.
Always annotate quick scene preview.

Since we started to use qquickshadereffectsource for the preview
images, the custom render mode is no longer displayed in the preview,
thus there is no need anymore to skip the annotation. This commit
removes the disabling.
Make measurement tool text slightly prettier.

The quick scene preview panel has a feature to measure pixel sizes.
This commit makes the text, that is displayed for it slightly
prettier. ("(123, 21) - (453, 23) -> 330.006px" instead of
"123, 21 - 453, 23 -> 330.006px")
Add qsTr to strings in quick scene preview.