fix header guards in rut-shim.h
inspector: support inspecting multiple objects

This adds support for inspecting multiple selected entities at the same
time by constraining the inspection to overlapping components and
forwarding changes made via the controls to the corresponding property
of all selected objects.
renderer: allocate text components preferred size

This makes sure RutText components will be allocated their preferred
size so that text is not left clipped to the original size based on
the default "text" property value.
Enable search for entities in the left search bar

We are no longer assuming the left area is strictly for searching assets
only and since we have a problem of not being able to easily navigate to
select certain UI entities (such as the camera) and we also plan to
utilize non-visual entities as part of our UIs it seems reasonable to
extend the search area to support searching entities that currently make
up the UI as an alternative way of selecting them.
Adds RutDragBin to support drag initiation

This adds a convenient way to enable drag & drop support by adding a new
bin widget with an associated child and payload object. If it sees a
button press that the child itself hasn't handled then it will start a
grab and visually show that a drag has started. During the drag an icon
will follow the mouse and widgets under the cursor will receive
_DROP_OFFER events that give them the opportunity to claim the payload
when the mouse is released.

To verify the interface we now support dragging and dropping assets from
search results onto asset-inspectors.
material: add inspectors for normal-maps/alpha-masks

We now show asset inspectors that support drag & drop and copy & paste
for the normal map and alpha mask properties of material components.