#16 [Debugger] Breakpoint synchronization

[Leif Frenzel]

Add a Haskell breakpoint marker type, add actions in the Haskell editor to toggle line breakpoints, add actions to the Breakpoints view to set line/column breakpoints and identifier breakpoints.

When launching in Debug mode, the current set of BPs must be transferred to GHCi and installed there. For each subsequent notification with the Breakpoint Manager, we have to communicate also to GHCi.

Problem: what happens when the user toggles a new BP while GHCi is already running? How can we get the BP in there before GHCi completely returns?