cmake: Modernize build system

Move to the modern, property-based way of using CMake. This
simplifies a lot of things:
 - No need to manually specify interface include dirs and libraries
   of dependencies
 - No need to carry around variables
 - Clear separation of local and global dependencies and definitions
 - No need for hacks to make the triple-executable thing work

Provide convenience functions to set up library and executable
targets. This will make it very easy in the future to split up
some modules, as well as move towards using shared libraries.

Use the modern, property-based way of using Qt, too. This removes
the need for the deprecated qt5_use_modules() macro, and a bunch
of manual things we had to do to get the compile flags right
for the different Quassel executables.
Also make use of CMake's AUTOUIC and AUTORCC to avoid a bunch of
boilerplate for dealing with .ui files and Qt resources.

Move the .ui files from their subdirectory into the qtui directory,
since the separation doesn't really make too much sense. We'd rather
want to split the QtUi module into smaller parts/libraries.

Remove a bunch of unnecessary stuff from the CMake files.