simplify prev commit
Restore 'Edit with basic controls' plugin UI functionality.

Regression was introduced in 40944574c

Compared to before 40944574c it is no longer possible to have
both custom and basic open at the same time.
On the upside window stacking and raise-to-top works properly.
Forward midi feedback from LV2 plugins and allow to chain LV2 midi plugins.
fix 5498 -- use note length from step-entry with midi keybd.
remember plugin-UI type (custom/basic) with session.

amend to 3.1-81-g1acf8bd
Remove unused boost/locale include.

boost/locale is only avail with libboost >= 1.48,
ardour's wscript checks for >= 1.39