New ITIP theme, ported to Grantlee

Use an improved ITIP theme which looks slighly more modern than the old
one. This is a port from vendor/intevation branch in kdepimlibs. It also
improves handling of invitation delegation.

As part of the patch the whole ITIP handler has been ported to Grantlee.
We slightly changed how attendees rows are generated, which also affected
some of the regular event view tests, this patch adjusts them accordingly.

Secondly to make all the tests pass regardless of available/installed
icon theme, we strip filename extension from <img src> attributes, because
while Oxygen uses .PNG, Breeze has .SVG icons, which causes the tests
to fail locally and pass on CI or vice-versa.