keymap: support combining shift & capslock with altgr

Add support for combining shift & caps lock keys with altgr so that
capital national characters can be written. There was some support for
shift+altgr in xrdp-keygen but it was never actually used.
dump-keymaps: generate 'pl' keymap
Update keymaps
Install and enable 0415/pl keymap
Merge pull request #178 from mgorny/altgr-keymap

altgr+shift/capslock support in keymaps