Use std::ostreamstream when input is not used

Instead of std::stringstream, which is both in and out by default.
Remove executable on `make clear`
Cache delimiter and color strings in StatusBar.cpp

For a better performance.
Fix includes of XKeyboard header

Should be "XKeyboard.hpp" instead of "XKeyboard.h".
Sort includes in StatusBar.cpp
Use for-each loop instead of std::for_each

The former is slower due to construction of the lambda expression, but
maybe it's not if a reference to the argument is used instead of a copy.
Use `.empty()` instead of `.size() > 0`
Optimize string operations in Network.cpp
Don't copy string in Field::getText() method
Use std::initializer_list for StatusBar::StatusBar