Use toolpallete and about icons from a GResource instead of installing.

* Makefile.am:
* Makefile_glom.am: Put the ToolPalette and AboutDialog icons in
  a GResource instead of installing them. Some icons still need
  to be installed as icons for the application and the mime type.
* icons/16x16/*.png: Move the toolpalette icons to icons/ as these
  were the only size ever used anyway.
* glom/utils_ui.cc: get_icon_path(): Make this return the resource
  path instead of a file path.
* glom/appwindow.cc: Load the about dialog icon from a resource
  instead of the filesystem.
* glom/mode_design/print_layotus/print_layout_toolbar_button.cc:
* glom/utility_widgets/layouttoolbarbutton.cc: Constructor:
  Load the icon from a resource instead of from the filesystem.