split: add IDE.SplitView to solve IDE specific problems

Especially the problem about window resizing. We need to preserve the pane sizes.
workspacebuilder: use IDE.SplitView instead of KDSplitView
split: s/IDE.SplitView/IDE.BaseSplitView
Merge pull request #68 from usirin/sidebar-resize

preserve file tree size when resizing
Merge branch 'master' of github.com:koding/IDE into collaboration