Use DockPanelSuite

[jng]

We have long avoided using DockPanelSuite due to P/Invoke calls in the code making the library unusable in Mono and have had to hack our own pseudo-docking component in its place.

But now, a patched version of DockPanelSuite exists ​here which stubs out the P/Invoke calls at the expense of drag and drop support (a feature we don't even use!). Initial tests show that this patched version works in both Windows and Mono with no showstopping bugs. So we should move to use this library instead and remove our existing pseudo-docking component.