Add libportal clipboard handling to the remote desktop.
Supports both directions (server to Wayland client paste,
and copy from the Wayland client back to the server)
for UTF-8 text, plain text, and PNG/BMP images.
The shared mime-type table, BMP/DIB and PNG encode/decode helpers, and
portal selection read/write live in a new PortalClipboard
helper used by both PortalInputCapture and PortalRemoteDesktop.
If the portal returns a restored RemoteDesktop session without clipboard
enabled (a token persisted before clipboard support existed), discard
the restore token and reconnect so the user gets a fresh permission
dialog. Otherwise the session would loop with a clipboard-less token
forever.
