added target for Pidgin plugin window_merge

I tried to put the installation instructions[1] into Makefile format.
This is quite hack-ish, but it works for now(TM).

1 https://github.com/dm0-/window_merge/wiki#gnulinux-rpm-based
added pidgin-window_merge to all
+trash-cli
target pidgin-window-merge now working under Docker and natively