selection: convert \r -> \n when reading clipboard data

This fixes an issue where pasting (using e.g. OSC-52) in client
applications that doesn’t do this conversion themselves, like tmux,
doesn’t work.

Closes #752

CHANGELOG.md

@@ -59,6 +59,8 @@
   state, foot was looking at **depressed** modifiers, not
   **effective** modifiers, like it should.
 * Fix crashes after enabling CSD at runtime when `csd.size` is 0.
+* Convert `\r` to `\n` when reading clipboard data
+  (https://codeberg.org/dnkl/foot/issues/752).
 
 
 ### Security