Get rid of Xlib on the keyboard indicator

Added a new dependency: [KGuiAddons](http://api.kde.org/frameworks-api/frameworks5-apidocs/kguiaddons/html/classKModifierKeyInfo.html). The KModifierInfo class is responsible for signalling modifier keys locks/unlocks.

Now the plugin is able to show the state of the Caps Lock, Num Lock and Scroll Lock at the same time (it couldn't before :expressionless:). However, now there is no option to set a custom key.