[WIP] Add ubuntu-app-themes to paths

[tsdgeos]

This way we enable snaps that plug into ubuntu-app-themes to have extra GTK/Qt5 themes available

If the plug is not connected the extra paths will just be ignored

Tested together with
https://code.launchpad.net/~aacid/+junk/ubuntu-app-themes
https://code.launchpad.net/~aacid/+junk/style-chooser-example-qt5
https://code.launchpad.net/~aacid/+junk/breeze-button-example-gtk

[didrocks]

Mind solving the conflicts with master? Then, we can proceed :)

[tsdgeos]

Doesn't make much sense to merge this unless someone takes ownership of http://bazaar.launchpad.net/~aacid/+junk/ubuntu-app-themes/view/head:/snapcraft.yaml and publishes it to the store.

And i guess someone should agree & probably document the "ubuntu-app-themes" name i chose.

[didrocks]

ok, I think I'll just close the PR and we can reopen as needed :)