Allow drag&drop conflict files to main window

While I was doing some conflict file resolving after automatic merge, I was thinking about faster way to open conflict files. Current way involves browsing the file using normal Windows Open dialog. Which is a bit tedious if you have multiple files.

This patch allows one to drag&drop the conflict file to main window. WinMerge checks if one file was dropped and then checks if the file is a conflict file. Yes, it takes a bit of time to do this check. But it should not be visible for user for most files.