Add central shell API component

Currently we have several locations and objects being exposed for the shell containments to interact with the shell and each other.

Expose all basic shell functions directly in one component to clean things up.

Usage would be something like:

import org.kde.plasma.private.mobileshell 1.0 as MobileShell

MobileShell.Shell.showTaskSwitcher();

Rather than currently:

import org.kde.plasma.private.mobileshell 1.0 as MobileShell

MobileShell.HomeScreenControls.taskSwitcher.show(true);