Rework channel labeling and add source picker to virtual-channel creation

Relabel every channel picker/header to "NAME (DEVICE/SIGNAL)" instead of
the previous "DEVICE/SIGNAL (NAME)" ordering, consistently across
channels_window.py (_channel_combo, ChannelPickerDialog,
ChannelPipelineBlock header), plot_builder.py, plot_config.py,
control_editor.py, and plot_window.py.

While touching each of those pickers, added the missing `.enabled`
filter that _make_picker()/_x_cb/_build_channel_picker lacked (the
default-layout builders already filtered disabled channels; these
manual "add channel to pane" pickers didn't).

Devices window Signals tab: removed the separate "Signal ID" column
(folded into the existing non-editable Device column instead of the
editable Name column, to avoid corrupting the in-place channel rename
feature that column already supports).

Channels window Virtual Channels pane: added a "Source" dropdown next
to "+ Add Channel" — picking a source pre-seeds the new derived
channel with it via DerivedBlock._add_src(); leaving it on the default
"none" entry keeps today's behavior of creating an empty channel.

Note: docs/ToDo.md's "Channels window UI" items referred to the *live*
UI (this window's PipelineTab + the Devices window's Signals tab), not
SignalsListTab in this same file, which its own module docstring flags
as dead/unused code kept only for old-profile compatibility — verified
by grepping for any instantiation of it (none exist).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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