Coding style
GUI: make it "(recent/unread)" instead of "(recent + unread)"

This definitely looks better than the old version. Sugegsted by Juliusz
Chroboczek -- thanks!
GUI: fix segfault in the FindBar dismissal

Because the underlying QWebView can be deleted more or less any time (for
example as a result of the user navigating away to another message), it is
really bad idea to keep it around indefinitely.

There was a reproducible segfault:
- open a message
- press ctrl+f, type something to execute a search
- navigate to another message
- dismiss the find bar

Nothing more besides using a guarded pointer is actually needed here; the checks
appear to be placed in all entry points already.
GUI: hide the FindBar when the underlying QWebView disappears

Looks like there were enough checks in place :), but in the form of asserts :(.
It's cleaner to just dismiss the search widget when the web view disappears.
GUI: improve the attachments widget

- Add a nice icon from the Oxygen theme
- Make it multiline to conserve space on loooooong MIME types like the
application/vnd.openxmlformats-officedocument.*
- Make it possible to select the attachment filename by mouse for copy-paste
GUI: use proper icons and human-readable description on Qt5
Add a comment about the QFont warnings