Merge the play and pause button

Instead of having two multimedia buttons, we can have the functionality with only one button with two states.
One state for play and other for pause.
Hence whenever they are called, a signal is sent to slot in MidiPlayer.qml.

Reviewers: sandroandrade

Subscribers: itsback

Tags: Minuet

Differential Revision: https://phabricator.kde.org/D1106