Allow pasting joined lines not to create empty lines

[eugenesvk]

• create two cursors with with copy_selection_on_next_line
• extend to selection to capture the whole line with the convenient extend_to_line_bounds (in the screenshot the extend_to_line_end command is just as convenient and doesn't capture newlines, but too often it's not as you have cursors from e.g. a search operation in the middle of a line, so extending is better than moving and selecting, and also I don't get the logic why the extra newlines would appear)
• copy both lines with yank_joined_to_clipboard
• paste with paste_clipboard_after
• see two extra empty lines created, though you'd expect the joined two lines to only have two newlines between them

Quick illustration

[dead10ck]

I believe this is related to / duplicating #1377. The issue is that the selection includes the newline characters at the end of the line, which you are inserting after the last newline under your cursor. I'd guess that shift-p would not do this.

[eugenesvk]

The other issue is about the first newline before the paste, which is not what I write about — my issue is with the two newlines that are created AFTER each line, and paste_clipboard_before doesn't change that. For some reason joining multiple selections doesn't just glue them, but inserts newlines in between (or maybe it happens on paste, not on join)