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The slip tool allows to move in and out point of a (cut) clip by a certain amount while position and size of the clip stay the same.

Related to #1069

Commit includes:
- basis for advanced trimming tools (e.g. trimming preview in monitor)
- make editing tool available as enum in qml
- add slip tool
- add tests for slip

Related to #1168

Fixes #1173

Appimage: abort if missing frei0r, avformat, vidstab or rubberband modules, warn on missing openCV and translations

(cherry picked from commit 226c550b)

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

Fix mix crossfade sometimes using wrong order (starting at 100% and ending at 0%) instead of reverse

opening older project file

Fixes #1162

CCBUG: 440218

Co-Authored-By: Anders Lund <anders@alweb.dk>

BUG: 440218

broken by f7f3fc21
BUG: 440414

CCBUG: 440867

Fixes #82

BUG: 408434