Clipboard should handle multiple clips

[dvc94ch]

After making an insertion a new keybinding context gets activated. This allows the next command to move forward or backward in the clipboard ring buffer. A command that is not or disables the context.

Unresolved questions:
Interacting with the system clipboard.

Possible solution:
Copying saves system clipboard in internal clipboard before replacing it's contents. Paste always inserts from system clipboard and next/previous use internal clipboard.

Possible solution:
CTRL-C, CTRL-V are used in both single and multi-mode for copying into and out-of theia. New copy paste commands are added for theia internal clipboard.

Relates-to: #1816