rdp: add clipboard support
This MR is made up of 2 commits. The first commit moves some of the clipboard implementation from vncview into the base remoteview class, so that it can be shared with rdpview. The second commit implements clipboard copy/paste support for text on the rdp plugin. Based on https://learn.microsoft.com/en-us/openspecs/windows_protocols/MS-RDPECLIP/fb9b7e0b-6db4-41c2-b83c-f889c1ee7688 and the FreeRDP example clients at https://github.com/FreeRDP/FreeRDP/tree/master/client