Change applicationName to 'quassel'

QCoreApplication::applicationName() should match things like our
data dir prefix and similar, so 'quassel' makes more sense than
'Quassel IRC'. This will be especially relevant in a KDE Frameworks
world.
Extend XDG_DATA_DIRS to include Quassel-specific directories

XDG_DATA_DIRS is used for example by Qt to find icon themes and other
application data. In order to avoid having to hack our own locations into
various places, we extend the variable at runtime to include them instead.

This is only relevant if KDE integration is not enabled; otherwise KDE will
specify a list of data dirs (this means that Quassel needs to be installed
properly for everything to work as expected if KDE integration is enabled).
Rework how icons are handled in the build system

In preparation of moving to native Qt icon theme support, this
commit reworks the handling of icons in the build system:

* Move all vital icons (i.e. application, channel and nicklist icons)
  into the hicolor theme
* Always embed the hicolor theme, so we always find the vital icons
  as a fallback
* Move the bundled Oxygen theme to oxygen/ and include metadata and
  index files, so QIconLoader can find this theme as a fallback
  (a system-installed theme will be preferred in the future, however)
* Ship all icon sizes rather than just the requested ones, so Qt can
  later pick the most applicable size
* Refactor the Oxygen import script accordingly
* Clean up the CMakeLists.txt
* Adapt resource initialization code
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).
Add command line parameter for overriding the system icon theme

Quassel uses many icons that are not part of the freedesktop naming
spec, but supported by KDE icon themes. If Quassel is run in
another environment, such as GNOME, which would select a non-KDE icon
theme, there may be missing icons.

In such cases, it is recommended to install the Oxygen icon theme and
start the Quassel client with '--icontheme=oxygen' to override the
system setting. Of course, you are free to choose any other theme as
well.

Note that oxygen is already selected as a fallback if the platform does
not tell Qt about its icon theme (for example, on non-Linux platforms).
Remove the obsolete Icon and IconLoader classes

Removing (hopefully) all traces of the now obsolete icon
loading support.