Model to provide easy access to KWin's Clients from QML

A new ClientModel is added which provides multiple different views on
KWin's Clients. The model is organized as a tree model supporting the
following levels:
* activities
* virtual desktops
* screens
* none

The levels can be ordered in whatever way one wants. That is the tree
structure can have an ordering of activities then virtual desktops or
the other way around.

In addition the model provides Exclusion flags to exclude clients of
certain types. E.g. it's possible to exclude all windows which are not on
the current desktop or all windows which are of type dock.

The model gets automatically updated whenever a Client is added/removed
or changes a state in a way that it should be excluded/included.

The ClientModel is not directly exported to QML. Instead there are
specific sub classes for certain common orderings. This solutions is
chosen to workaround some limitations of QML. The initial idea was to
use a property taking a list of the levels, but this doesn't work because
we are not notified when the QDeclarativeListProperty changes.

Currently the following models are provided to QML:
* ClientModel -> no restrictions
* ClientModelByScreen -> ordering by screen
* ClientModelByScreenAndDesktop -> screen, then desktop

These can be used to get all Clients:
ClientModel {
}

Or to get the classic Present Windows on current desktop:
ClientModelByScreen {
exclusions: ClientModel.OtherDesktopsExclusion | ClientModel.NotAcceptingFocusExclusion | ...
}

Or to get the classic Present Windows on all desktops:
ClientModelByScreen {
exclusions: ClientModel.NotAcceptingFocusExclusion | ...
}

Or our well known desktop grid:
ClientModelByScreenAndDesktop {
id: desktopGrid
exclusions: ClientModel.NotAcceptingFocusExclusion | ...
}

To support filtering as known by the Present Windows effect one can use
a ClientFilterModel, which is a QSortFilterProxyModel filtering on
window caption, role and class:
ClientFilterModel {
id: filterModel
clientModel: desktopGrid
filter: filterItem.text
}

In case it's a tree level obviously QML does not support this correctly.
So we need to use a VisualDataModel:
VisualDataModel {
id: clientModel
model: filterModel
Component.onCompleted: {
clientModel.rootIndex = modelIndex(0);
clientModel.rootIndex = modelIndex(0);
clientModel.delegate = thumbnailDelegate;
}
}

As we can see, the rootIndex has to be set to the level which contains
the Clients. Also it seems to be important to create the delegate after
the model index has been set. The idea is to have only one ClientModel
and multiple VisualDataModels if multiple views on the data is needed.

The model has been tested with a painful modeltest session. It looks good
so far modulo the listed limitations and that modeltest is not liking
closing Yakuake in the ClientModelByScreenAndDesktop setup, though it
works fine in real world testing.

REVIEW: 109604