Adding "Set note pan" statusmessage + other changes in the note properties ruler and pan value input

[oddtime]

This message was missing
Allows to know if pan is Right or Left (since there's no indication on the note editor ruler) and the value.
Minimal solution to issue #948

made some messages translatable
reduced size of buffer strings

The display format of the pan value can be discussed, right now is from 1.00L to 0.00L, C, from 0.00R to 1.00R.
Another fashion could be: L100 to L0, C, R0 to R100

// edit: "L","R" marks on not pan editor background added

[trebmuh]

LGTM.
Is there any chance for the "R" & "L" to be translatable? or maybe it's already the case?

[oddtime]

Not now but I can do it

[trebmuh]

It'd be awesome.

[oddtime]

See #948
Unfortunately the text is fixed on background (it scrolls with it) but for now we can live with it.

[oddtime]

I have changed the code to input pan from GUI calling the pan law methods (the ones in Sampler class). This solves #1167.
Maybe a factorization here would be useful (similar lines are coded in both "mouse move" and "mouse wheel" functions...), I would leave this as it concerns all the note properties if I am not wrong.

[oddtime]

Actually I am doubting the necessity of still having pan_l and pan_r in Note and Instrument classes, I opened an issue: #1242
I am tempted to deprecate them and get a one to one correspondence between the widgets value and the new unique pan member