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:

Seek_durations

Closes #1685 (closed)