Add a CMake wrapper for finding indicate-qt

Because CMake's feature_summary works only with dependencies found
via find_package, it makes sense to have a wrapper around the
PkgConfig call used to find indicate-qt. This is very basic and does
not support all of the usual options (such as checking for a particular
version), but sufficient for our needs.
Don't make EventType::KeyEvent depend on HAVE_QCA2

This enum value can be known even if we don't have encryption support.
Remove unnecessary include

CoreApplication does not in any way depend on KApplication.
Fix finding a custom Qt4 via -DQT_PATH

This CMake option has recently been renamed from QT to QT_PATH; we
really should actually use the new name...
Completely rework the dependency handling in the build system

Traditionally, optional build dependencies are handled in CMake via
offering options and conditionally searching for the needed packages.
This leads to having lots of options, boilerplate for telling the user
which features are enabled or not, unclear handling of automagic
dependencies, and so on.

These days, CMake offers a much nicer way: feature_summary() and friends.
To make use of this, one calls find_package unconditionally and then sets
properties on the package that tell CMake if the dependency is optional
or required, its purpose, a description of the package etc. At the end
of configuration, CMake then displays a nice summary of all the packages
searched for and (not) found, as well as other features.

Quassel now makes use of this modern way of specifying dependencies, replacing
most of the former options (they're still there, but unused now, and will
be removed soon). Note that one can still disable an optional dependency Foo by
passing -DCMAKE_DISABLE_FIND_PACKAGE_Foo=TRUE to CMake.

I also took the opportunity to radically clean up and fix how we handle
dependencies throughout the build system:

* We no longer set an extra CMake variable to indicate that a package was
  found, but use the Foo_FOUND variable directly
* Compile definitions and include directories are no longer set globally
  in the root CMakeLists.txt, but in the Quassel modules where they're
  actually needed; this should speed up compiling ever so slightly and
  generally make things much cleaner
* Libraries are now linked directly to the Quassel modules that use them,
  rather than to the executable. This prepares us for making the modules
  shared libraries at some point, and is anyway The Right Thing™ to do
* Fully support Qt5 in the build system, including finding the proper version
  of dependencies where appropriate and available

Note that the changes to the build system are not complete yet, and some things
may be broken now. Note also that while Qt5 is now supported in the build system,
the code itself still requires many changes to actually compile with Qt5,
so don't even try.
Make SSL and syslog support non-configurable

Both features have no external dependencies; SSL support requires Qt
to be built with SSL support, and syslog support requires the existence
of the syslog.h header. It makes no sense to make this configurable,
as no additional libraries or other dependencies are pulled in.

Instead, just add them to the feature_summary depending on the prerequisites
being in place.
Remove obsolete CMake options

Now that the new dependency handling is in place, we can remove several
of the CMake options we used to have. If you don't want automagic
dependencies, and want to disable one even though it is installed, you
can use -DCMAKE_DISABLE_FIND_PACKAGE_Foo=TRUE for ignoring the Foo package.
Update INSTALL file

This hadn't been touched since 2007, and with all the recent build
system changes, I guess it made sense to adapt it to reality.