Improve task selection for time sheet generation

[AndreSomers]

When you generate a time sheet, you can select which tasks to include. I find the UI for that to be very uintuitive. You cannot select whole subtrees in one go for instance. Why are you not simply using checkable items in the tree? I posted an proxy model to add selection to tree models ages ago; see https://wiki.qt.io/QSortFilterProxyModel_subclass_to_add_a_checkbox, google for the actual code as gitorious seems to be dead).

What I am most interested in however is creating a standard filter "billable".