When double-clicking to edit a fader value three button press
events are created:
1. GDK_BUTTON_PRESS
2. GDK_BUTTON_PRESS
3. GDK_2BUTTON_PRESS
The first two start an Editor Drag, which later crashes in
Editor::mid_track_drag() gtk2_ardour/editor.cc:7067
This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
This follows 5a41ca8fdf and df1e6fda2d.
Even though this kind of precision is rarely needed, rounding
to a single digit is inconsistent with various places that display
more than one digit, or allow more accurate control.
