Use Qt's native icon theme support rather than our own

For the longest time, we've had our own icon loader (or used KDE's)
in order to provide rudimentary icon theme support. However, since
2009, Qt has had native support for loading icons from themes. In recent
version (due to platform integration etc.), proper icon theme support
has gotten much more complex too... things like theme inheritance,
proper fallback policies, specialized states and so on have never been
supported by our simple custom icon loader.

This commit switches all icon loading to QIcon::fromTheme().

The downside is that we have little control about Qt's icon loader,
which may cause issues with some of the non-standard icons we ship.
In case of the native Qt icon loader being used, we define fallbacks
that should find the extra icons. If you use Qt5 and have the
KDE Frameworks integration plugin for Qt enabled, these fallbacks won't
work unfortunately. If you see missing icons, you may have to install
Quassel's 'hicolor' icons into /usr/share/icons/hicolor so they get
picked up.

For non-Linux platforms, we recommend using the bundled Oxygen icon
set (this is the default).