Implement a Base system for plugins within Konsole

This commit introduces a plugins sytem for konsole, so we can
extend the application without touching the core. Currently
there's a testcase plugin (in the next commit) that handles
ssh connections and will serve as a base introduction on how
to write plugins for konsole.

Things we need to keep in mind when developing a plugin for
konsole:
- We can have more than a mainwindow, so if the plugin creates
widgets, it needs to create the widget per window
- We can have multiple tabs / multiple TerminalDisplays so always
check what terminal display you are triggering the action (using
activeTerminalDisplay())

From the mainWindow you can get the activeTerminalDisplay, from
the terminalDisplay you can get the Session, the VT102Emulation and
the Screen.

Then you can go crazy.