This is incorrect and we should always the right constructor instead

Action stack of parent operation should not record the
actions being undone in case rollback is done on a nested
operation.

Fixes #2311

Add a method check_selection to keep a check on selected attribute
of clips and selection.

This way the user won't be confused about what's selected.

Fixes #2409

Found via `codespell -q 3 -S *.po,./po -L amin,dur,iff,ith,que,ressources,splitted,superceded,uint,unselect`

Fixes #2418

Fixes #2419

The position passed to GES was incorrectly calculated for both left and right
handles. For the left one a 1px jump was caused by `__drag_start_x` being
between 0 and 1. For the right one, a jump of a few pixels was caused by
ignoring both `__drag_start_x` and the width of the handle, as the right edge
position must be passed to GES.

Fixes #2295

We were forcing 1 channel because of the way the previewer was
implemented.

Fixes GNOME/pitivi#2338

Quicktime is more used in the industry and GStreamer better handles
it and allows us to be frame accurate, matroska has issue with
frame accuracy seeks.

Fixes GNOME/pitivi#2324

Fixes #2369

The method is applied to the project video framerate, to the audio rate,
and to the audio channels number. In addition, in the Render dialog it's
applied to the muxer and to the encoder(s).

- The audio widgets should always succeed as we don't allow custom
  values.

- The muxer is allowed to fail as it means it's missing from the current
  system, and maybe the user wants to render using a different one.

- Similarly, we pick the first available encoder if the preferred one
  cannot be found.

- The video framerate is allowed to fail as we allow setting a custom
  framerate.

Fixes #2425

A warning button is displayed in the headerbar when soft deps are missing.
The button shows a popover with details.

Fixes #2215

This way GES will be able to compute new positions of the source
(if the aspect ratio was not changed)

See https://mail.gnome.org/archives/desktop-devel-list/2020-February/msg00055.html

First version shows an overview of the UI sections.

Fixes #2361

Fixes #2315