* Adds `CopyTo` and `PasteFrom` assignments that specify the
destination/source.
* Adds `default_clipboard_copy_destination` and `default_paste_source`
config options that specify the default destination/source for
existing `Copy` and `Paste` operations (for @bew)
* Deprecating `PastePrimarySelection` in favor of `PasteFrom`.
* Added `CTRL-Insert` -> `Copy` (for @Babar)
Aside from the new key assignment, these changes shouldn't change
the default behavior, but do make it easier to consider changing
that in a later commit.
They should allow for example:
* Set `default_clipboard_copy_destination = "PrimarySelection"` to
prevent populating the clipboard by default when using the mouse.
* Overriding the CTRL-Insert, CTRL-SHIFT-C to explicitly populate
the clipboard
* Set `default_paste_source = "PrimarySelection"` for middle click
to paste the selection.
* Overriding SHIFT-Insert, CTRL-SHIFT-V to explicitly paste from
the clipboard.
refs: #417
