Add support for specifying COMMUNI_INSTALL_XXX

qmake or environment variables:
- COMMUNI_INSTALL_PREFIX
- COMMUNI_INSTALL_BINS
- COMMUNI_INSTALL_PLUGINS
- COMMUNI_INSTALL_ICONS
- COMMUNI_INSTALL_DESKTOP
Merge SystemMonitor to the app - kill MonitorPlugin

It was annoyingly coupled with MainWindow, and creating too
many instances - one per connection plus one for the window.
PluginLoader: blacklist obsolete monitorplugin
Fix plugin loading on Windows
Speedup plugin loading on Linux
WIP: quassel support