Union in Plasma 6.7

[Arjen Hiemstra]

Hi,

With Plasma 6.7 we will have a release of Union, the new style engine that 
we've been working on for a while now. The current state is that we have a 
fully implemented QtQuick Controls style along with a Breeze reimplementation 
in CSS. Some of us have been daily driving it for a while now and while there 
are regressions, most of these are to the extent of slightly different paddings 
or mismatching colors.

However, we cannot test all applications out there on our own, so we may have 
missed things. We really need more testing to figure out if there is major 
breakage and if so, how things break.

This isn't really possible without a more general rollout of Union, so we had 
some discussion yesterday about what exactly we want to do for 6.7. I think 
there have been three options floated about:

- We switch the default QtQuick Controls style. Since only Breeze is currently 
implemented in Union, switching it without some fallback would annoy people 
who don't use Breeze QtWidgets, so the current idea is to only use it when the 
user has selected Breeze. The main drawback is that there's no simple escape 
hatch for if an application does break to such and extent that it is unusable.

- We add an option somewhere for users to opt in using Union. The advantage is 
that this could be fully opt-in. It could also explain more about what it is 
and what the state of things is in UI. The disadvantage is that for most users 
it is going to be a meaningless option that they won't fully know what the 
implications are, since it's mainly a technical option.

- We do not do anything and rely on users setting `QT_QUICK_CONTROLS_STYLE` to 
Union. This I think is the worst outcome, as it makes it quite unlikely that 
many people use it, which means we don't get the testing coverage for Union 
that we need at this point.

I know this discussion should have been started sooner, but the soft freeze 
date kinda snuck up on me, so here we are.

- Arjen