This patch refactors the checks for inclusion of TabBox clients in the client list. All the checks have been implemented as dropdowns. Quoting Martin, the checks are:

* Desktop: Windows From All Desktops, Windows From Current Desktop
* Activities: Windows From All Activities, Windows From Current Activity
* Applications: All Windows From All Applications, Only one Window per
Application, Only all Windows of Current Application
* Minimized: Ignore, Minimized windows only, Exclude Minimized Windows
* Desktop: Don't Show Desktop, Show Desktop to minimize all windows
* Multi Screen: Ignore, Only Windows from current screen, Exclude windows from
current screen

Each of such checks has been implemented as a private method of TabBoxHandlerImpl. They are called by TabBoxHandlerImpl::clientToAddToList, while ClientModel::createClientList only handles the switching mode (FocusChain/StackingOrder) and whether a desktop client should be added to the list to minimize all the clients. In TabBoxHandlerImpl::clientToAddToList, I left the following line:

addClient = addClient && current->wantsTabFocus() && !current->skipSwitcher();

as I am not sure if it is needed or not.

The UI is a bit crowded now, but I would leave a UI refactor for a future commit.

Desktop, Minimized, Show desktop: work fine

Multi Screen: could not test, as I do not have a multi screen configuration

Activities: works fine. However, when using the present windows effect as task switcher, the previews of the windows on other activities are not visible. They are actually there, because moving the mouse to the area where they (presumably) are makes the close button appear and allows to activate them. When using a layout based switcher, thumbnails of windows on other activities are correctly shown. So, I guess, it is something unrelated to the checks I made.

Applications: the new case I implemented is "Only all Windows of Current Application" (others work fine). I tested it with KDevelop+About KDE+About Kdevelop, and it works fine. It doesn't work with the three windows opened by Gimp, in the sense that only one window (instead of all of them) is shown. Not sure if my check is incorrect, here.