Addresses #261.

Previously we'd invalidate the entire slot only if the item was under a
DnD operation.  Now we unconditionally invalidate the union of the slot
rect and the item's calculated extents, if any.

This might cause a *little* more drawing than necessary in some cases,
but fixes some intermittent drawing glitches when moving icons around.

Also chain up in size_allocate, since the parent impl does most of what
we were doing anyway.

The changes in a style update might affect where we draw the icons, but
shouldn't affect the pixel data in the icons themselves.

Closes #261.

There are far too many instances of gtk_widget_queue_draw() all over, I
suspect to paper over redraw issues.  Removing all of these (and
replacing them with individual or all icon invalidation) should be fine
to do.  There's still the possibility that some areas won't get
invalidated when they need to be, but they should be fixed when found.
The only time we should `_queue_draw()` is when the backdrop changes.

Closes #284.

Closes #260.

Closes #261.

Closes #291.

There was really no need for this, as the concrete impls were listening
to `notify::workarea`.  At the time, this just wasn't hooked up
properly.

Again, icon view should not be concerning itself with this; manager
needs to size and place the icon view in the correct position.

(Which we never do!)

All of these various changes should just be handled in one place; the
icon view itself should perhaps not have to worry about it, and
something external should place and size it.

This leaves screen-changed alone for now, but I'll revisit that soon.

Closes #289.

It currently isn't being used at all, and the icon view shouldn't be
concerning itself with that anyway.

Also sets opacity of minimized app icons to 60%.

Closes #272.

This not only was breaking the cursor change to 'hand' when in
single-click mode, but it was breaking the style change on hover, and
making DnD act a little... weird.

Closes #292.

263 translated messages.

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263 translated messages.

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Closes #289.

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159 translated messages, 104 untranslated messages.

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263 translated messages.

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247 translated messages, 16 untranslated messages.

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263 translated messages.

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263 translated messages.

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263 translated messages.

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263 translated messages.

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242 translated messages, 21 untranslated messages.

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