cc-util: add a function to get vendor-prefixed app infos

We're going to use this from the Notifications and Search panels.

[endlessm/eos-shell#2756]
notifications: use the vendor-prefixed app info for display

[endlessm/eos-shell#2756]
notifications: force GTK_ICON_SIZE_DIALOG icon size

Force a pixel size too, so that we can work around applications not
shipping icons at all sizes.

[endlessm/eos-shell#2756]
search: use the vendor-prefixed app info for display

[endlessm/eos-shell#2756]
search: force GTK_ICON_SIZE_DIALOG icon size

Force a pizel size too, so that we can work around applications not
shipping icons at all sizes.

[endlessm/eos-shell#2756]
Merge pull request #8 from endlessm/shell/2756

Shell/2756