Improve timeline pasting UX

Fixes #2529 (closed).

Adds new behaviour to the clip pasting process: clip(s) are now selected immediately after pasting and timeline automatically seeks to their end - feels more intuitive for people coming from other video editors.

Doesn't fix the failing paste issue discussed in #2554, but clears the clipboard after a fail occurs as a temporary measure, disabling the paste button (so there's at least this way for the user to notice) to prevent further invalid pastes.

Adds testing of the selection logic to the existing copy-paste test. Decided to omit the seeking part since existing tests are based on the timeline position being mocked anyway.