Importing the backported MIME module from Qt5

This is based on https://github.com/pasnox/mimetypes-qt4.git commit
0c68b5527383e9616b0f13b2e4322badbde6c7fa.
Simplify and fix the standalone build
Make the header usable -- this is not a system header
Use the backported QMimeType on Qt4
Merge branch 'mimetypes-qt4'

Thanks to Filipe Azevedo Ternois for this backport.
Importing a set of icons from the Oxygen theme

They are from KDE Oxygen (I guess that the already present icons come from
there, so that's where I took them) which is released under the LGPL (see
http://www.oxygen-icons.org/?page_id=4).
LineEdit: a QLineEdit with an integrated button for clearing the input

refs #66
Importing the configurable shortcut manager

refs #565
GUI: use shortcuts from the configurable shortcut handler

refs #565
Fix a build warning (-Wmismatched-tags)
GUI: Let's work without a dedicated Settings menu for now

The generic application settings are already somewhere else, so let's not
introduce a top-level menu just for keyboard shortcuts.
GUI: don't provide a misleading icon for the "Reply by Guess"
qt5: Fix includes

In qt5, QtGui got renamed to QtWidgets. In order to prevent relatively useless
#ifdefs, it's better to skip the module name altogether.

I've also sorted the headers alphabetically (the removed module prefix changes
the order) and removed <QDebug> which is not needed anymore.
qt5: fix build
Merge branch 'shortcuthandler'

refs #565
Move Glad to a more prominent place now that he's contributed a big piece of code
refactoring: prevent duplicate file names
Composer: use Qt's MIME type facilities when attaching files