Remove deprecated OnlyShowIn from desktop action

Support for OnlyShowIn in Action groups has been removed from the
desktop entry specification according to [upstream bug] [1]. Starting
with version 0.22, desktop-file-validate warns about it asking for a
little bit of QA love.

[1]: https://bugs.freedesktop.org/show_bug.cgi?id=66712
Extract terminal display and management functionality to a separate widget

This is an important first step towards tabbed/split terminals.
Merge branch 'desktop' of https://github.com/ferki/finalterm
Do not store stream elements anymore