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
Tabbed and split terminals

A custom widget, NestingContainer, has been implemented
that allows for arbitrary arrangements of terminal widgets,
recursively combining tabbed and split views.
Tell CMake this is a C project to prevent checks for a C++ compiler
Update instructions for Fedora
Merge branch 'readme' of https://github.com/TingPing/finalterm
Update SRPM link and about dialog credits
Fix termlet installation directory permissions

There appears to have been a change in CMake causing directory
permissions to be preserved as well if the USE_SOURCE_PERMISSIONS
flag is used. This caused the "Termlets" directory to be readable
to root only, making the termlets inaccessible to Final Term
(first observed in Fedora 20).
Use precmd and preexec for prompt and command detection

Final Term now makes use of Glyph Lefkowitz's script from
http://glyf.livejournal.com/63106.html to emulate zsh's
"precmd" and "preexec" functions, directly extracting the
executed command from bash's history.

This fixes a number of bugs in command detection and enables
things that would have been near impossible using the previous
approach, such as correctly detecting multi-line commands.

Also, since zsh already supports precmd and preexec natively,
this suggests a natural route towards zsh support for Final Term.
Display command return code in user interface
Implement terminal commands

Terminal commands are freely configurable commands that are
entered through and interpreted by the shell but executed
by Final Term.
Use gee-0.8 when unity
fix #208
Arrive in 2014; update about dialog credits