SettingsController: Rework custom seek duration option
This merge request a follow-up from !1078 (closed):
- Rebasing the master branch and fixing the compilation issues.
This merge request also brings a patch this refactor, fixing:
- The custom seek duration display in both the video and audio players.
- Cosmetics changes in order to improve the readability of the files modified.
Here is a screen recording of the seek durations displayed with 5 seconds on swipe and 10 seconds on tap:
Closes #1685 (closed)