FSWidget: Implement select and deselect files

Select files by CTRL+k, deselect everything with CTRL+k (for kill).
Select all with the global shortcut CTRL+a.

Learned some interesting things: First, QAction shortcuts won't work if
you don't add them to any actions(), even if it has a parent. So
subclass QTreeView for the context menu and add an InvisibleAction enum.
Don't show the QAction if it has that flag in data(). This has a nice
side effect: the context menu isn't shown any more when you right-click
outside the FSView.

Second: The debugger was quite confused with equal SLOT names in
different classes when using the new connect syntax. It dropped me off
in totally wrong classes, confusing me at first, too :(

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