Shortcut to seek/scrub audio files

Summary:
Assigned Shift+Left/Right keys as shortcut to seek/scrub audio files (by 10 secs). Having shortcuts to seek/scrub is a good to have feature besides using the mouse.
The patch was in response to the bug 397523 which mentions to use Alt+Left/Right keys. But, as Alt+Left keys are used by ViewNavigation.qml Shift+Left/Right keys were assigned as shortcuts.
As far as fast forwarding is concerned, Elisa doesn't support it yet. So, no shortcuts were introduced.

BUG: 397523
FIXED-IN: elisa 0.3.80

Reviewers: Elisa, ngraham, mgallien

Reviewed By: Elisa, ngraham, mgallien

Subscribers: mgallien, ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D18017