Splice mode: exclude newly-added regions from being shuffled

When adding regions in splice mode, exclude the region being added from
possibly being shuffled. I don't know whether this might have some other
adverse effects, but it fixes an obvious defect in 'Splice' mode where
newly-recorded regions 'jump' to the playhead position when recording
stops, and since splice mode is pretty much broken anyway, I don't think it
can make matters any worse.
Splice mode: fix comment typo
Ripple mode: basic implementation

Add a value for Ripple to EditMode enum.

Add Ripple edit mode to edit mode dropdown, by adding it to the
Editor::build_edit_mode_menu() helper function, and remove the old code that
added items to the (now unused) Editor::edit_mode_strings.

Add the regions that should be affected by the drag to RegionDrag::_views so
that the drag carries them along automatically.

Use a copy of the RegionList in Playlist::core_ripple(), since bad things
happen when iterating over regions and they get moved around in the list.

Handle rippling in removal of regions from playlist.

When dragging in ripple mode, exclude all regions that lie before the
original start position of the selected regions being dragged from
rippling: this is what Mixbus does.

Make editor dragging respect snap-to settings, by using the existing
compute_x_delta() function, which did almost the right thing. Move setting
of _last_frame_position out of that function so all ripple-dragged regions
can move.

Ripple when dragging from region list: even though Mixbus doesn't do this, it
seems like a good idea.

Prevent multi-track selection being dragged across tracks, by making
RegionMotionDrag::y_movement_allowed() virtual, and overriding it in
RegionRippleDrag to forbid dragging of selections containing regions on more
than one track to dofferent tracks in ripple mode.

Remember which TimeAxisView a ripple-mode drag that's allowed cross-track
drags started from, so that the effect of rippling regions after any region
that's dragged off that track can be undone.
Ripple mode: tidy up

Remove a load of debug output and dead code.
Move implementation of RegionRippleDrag::y_movement_allowed() out of
header into .cc file.
Ripple mode: ripple when deleting ranges.

Make delete, cut and clear of a range ripple the appropriate playlists.
Don't clear the clipboard when deleting things

Deleting regions (or ranges, or anything) shouldn't affect the contents of
the clipboard - only CutCopyOp::Copy, CutCopyOp::Cut and CutCopyOp::Clear
should do that.
Ripple mode: ripple all after start of deleted range

When deleting a range in ripple mode, ripple everything after the start
point of that range backwards by the length of the range, so that
newly-created regions immediately at the range end move back to the range
start.
substantive changes to the logic and safety for naming of (audio/MIDI) sources, especially when created via import
fix crash recovery: add new constructors to SndFileSource, AudioFileSource, add a new SourceFactory method and finally tweak
AudioDiskstream::use_pending_capture_data() to create both the required whole-file and the in-playlist regions
fix issue with track-deletion when the monitoring section is used.
clean up after monitoring-section on session close
additional DEBUG_TRACE message for canvas current item selection
invisible items/containers should not add their children to "items-at-point"
use Editor::effective_mouse_mode() inside Editor::button_selection(); clean up special-case of fade in/out handles and mouse mode
remove canvas cursor debug output
make inactive group tab color be themeable and update it dynamically
make verbose cursor respond dynamically to theme changes
move contrasting_text_color() into ArdourCanvas
allow Canvas::Text to have outline drawn in a contrasting color (based on actual color)
verbose cursor text is always outlined