Add unread mail dashboard widget

[st3iny]

Ref #3742

Adds a second widget for showing unread mails from all inboxes. Extensively reuses the the code from Julius.

Here is a screenshot (mails are blurred for privacy reasons):

[st3iny]

Apparently, the coverage shrinks by 0.03% because the dashboard widgets are untested.

@ChristophWurst I can add tests if you want me to but I don't think testing the dashboard widget is that useful.

[ChristophWurst]

👍 excellent code. Thanks a lot 🙏

[jakobroehrl]

@st3iny Good job!
What about showing favourite mails in this list, too?

[ChristophWurst]

Apparently, the coverage shrinks by 0.03% because the dashboard widgets are untested.

@ChristophWurst I can add tests if you want me to but I don't think testing the dashboard widget is that useful.

Nono, that is fine. I wish there was a threshold to set until which it's fine to have the coverage decrease.

[GretaD]

All is good, the error i reported was not because of this PR. Thanks again Richard

[GretaD]

@st3iny I would propose for consistency to show the unread messages on dashboard as bold. But we can merge this one and do that in another follow up PR.

[GretaD]

@st3iny Good job!
What about showing favourite mails in this list, too?

Please open a feature request about this, so our design lead can have a look. In general we like to split features (or/and bugs) for better reviewing and possible regressions.