ui/crumbs/hig: make the layer selection dialogue more closely match design

ui/crumbs/hig: make the layer selection dialogue more closely match design

The layer dialogue design includes in-line remove/delete widgets next to
the layer path in the tree view for all layers other than the meta layer as
well as an in-line notice that the meta layer cannot be removed.

This is achieved  in this patch through the use of custom cell_data_func's
for the treeview to render the meta layer differently and a custom
CellRenderer implementation, CellRendererPixbufActivatable, which renders a
pixbuf and emits a clicked signal when the user clicks on it.

Fixes [YOCTO #2083]

(From Poky rev: 83c96b7a0ec4412716090098385a665045909a9a)

(Bitbake rev: b2d8f28c0059992200457a15aef8be09e015a5dd)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Shane Wang <shane.wang@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>