Offser a dropdown menu to select the DbgEng path

[xusheng6]

Right now we have the "debugger.x64dbgEngPath" setting that allows the user to type in an alternative path for the x64 dbgeng path. It works like this, if this path is not provided, then the debugger uses the one that it ships (the default), and when this path is provided, it is used. This works good so far, but it is not convenient to switch between the default one and a custom one, especially in light of #552, with which we will help the user download and setup the WinDbg installation which supports the TTD. But it is not always the best choice, because per my testing, it is slightly slower than just using the raw dbgeng. So the chances are the user would be switching back and forth between the two when necessary

I figured it would be best if we can change this setting to a dropdown menu and user can select which one they want to use