Merge pull request #491 from PeterBjuhr/sessions

Trying to wrap up and close issue #470
pot upd, nl translation upd
use bitwise and, not logical
always save the session-specific include path, otherwise it gets lost when the user toggles between enable/disable
The duration attribute of Durable, Partial, Tempo and Tremolo becomes a
normal tuple(base, scaling). An actual written duration results in a
Duration child appended to the item the duration is written after.

See issue #492
hopefully fix #493
append the number
change Tempo: values are not stored in attributes anymore
methods remain the same
add a vary basic plaintext() method that returns a text value for the node (e.g. string or markup)
new method defaultfilename
(use only when there is no filename yet)
provide a default filename (implement and close #472)
Merge branch 'master' of github.com:wbsoft/frescobaldi
the fields are only specified in one place
handle \\concat correctly
update chlog
open default viewer upon successful compile

implement and close #435
upd chlog
convert-ly: improve diff highlighting, dont apply dialog when clicking save button
close #436
Don't disable Copy global path button after use #470
Consistent syntax