SVN_SILENT made messages (.desktop file)
cleanup: remove unreachable code
GUI: QTabBar is nicely reachable via member function
Refactor the way how we find preferred item within multipart/alternative
GUI: expand previewable attachments upon first click on the attachment icon

The motivation is to make expanding of previewable attachments smooth, but at
the same time do not interfere with the way how the button has worked so far too
much.

When an attachment cannot be previewed, nothing changes and everything works as
it used to work before.

When a preview can be shown, however, clicking on the icon of the attachment at
the time the preview is collapsed will simply show the preview. When the preview
is shown, however, clicking any part of the icon will still bring up the menu as
it used to work previously.

This is an alternative implementation to review #117068.

v2: Use another approach for determining what action to take based on the actual
state of the preview in order to prevent duplicate connections. Thanks to Thomas
Lübking for pointing out a possible problem here.

BUG: 332586
REVIEW: 117295
GUI: show a warning for "unknown" multipart/alternative parts

v2: Also provide simple captions for text/plain and text/html MIME types, and
simplify the code by calling setTabIcon instead of repeating the call to
addTab().

BUG: 332950
REVIEW: 117335