Use `cond' instead of (if ... x (if ... y z)).

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Add marker-property weechat-text to mark text content of a line.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Add `weechat-line-text'.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>