Change Signature: Adds a toolbar with expand all / collapse all buttons.

I had to change SINGLE_TREE_SELECTION to DISCONTIGUOUS_TREE_SELECTION so a user can select everything in the tree with a few keystrokes (invoke expand all through the toolbar and then press CTRL+A to select everything and afterwards press SPACE so every checkbox is selected)
CheckboxTreeBase.CheckPolicy was called with uncheckChildrenWithUncheckedParent=false but it should be true, because it doesn't make sense to have a caller catch an exception a callee never throws
This was accidentally added in revision 7055e200487d8613aba30fcf525c7dfce323fca7

[x] m3         // throws exception
    [ ] m2      // callee of m1
        [x] m1 // caller // this was possible
Merge branch 'master' of https://github.com/lehnerj/intellij-community