Adds the ability to connect and disconnect strips in the VSE.
- Connected strips have an icon indicating their status, and attempting
to select one connected strip selects all other connected strips in
that chain.
- If the user attempts to connect a strip that is already connected to
other strips, that strip will disconnect itself from others before
connecting to new strips.
- Preview selection also works in bulk if multiple video strips are
connected together in the timeline.
- When adding new strips from the Add menu or the File Browser, strips
from the same file are connected by default. There's an option to
turn this off in Editing > Video Sequencer user preferences.
- It is possible to individually tweak strips/handles and ignore
connections with Alt+Click.
- This shortcut overrides the old keymap item for "Linked Handle"
selection. The property still exists if people want to use that
shortcut for its old purpose.
- To make sure that connections remain valid even after duplication,
I've added a condition to `seq_new_fix_links_recursive` that also
updates connections using the `seq->tmp` var. (A note -- I've updated
the comment for this field in `DNA_sequence_types.h` because the var
is only used for duplication now. It was once present in
`select_more_less_seq__internal` to be used for linked selection but
is gone now).
- There are also functions to cut one-way links and make sure that
all strips are bidirectionally connected after duplicating.
Pull Request: https://projects.blender.org/blender/blender/pulls/124333
715129bf5b