Implement new context view with metadata

Summary:
enlarge the context view to fit new content

allow ManageHeaderBar to export a databaseId property for current track

in the detailed metadata view compose the property name with i18nc

partial upgrade of the database to better handle some missing metadata

do not always put a value for disc number when the metadata is not here

add back indexes when recreating Tracks table

allow comment of tracks to be null

store some metadata only if they are really defined

only really return data on tracks if they are not null in database

allow to inherit from TrackMetadataModel to filter properties

remove obsolete methods from TrackMetadataModel

introduce TrackContextMetadataModel to display metadata in context view

fix TrackMetadataModel to not create multiple connections

provide new context view with metadata from currently playing track

remove useless debug prints

fix size of the cover in context view

when showing ContextView, fix width to not overflow

add the filename at bottom of context view

fine tune the display of context view to be with regular spacing

Test Plan: context view mostly look like the design done by @januz

Reviewers: januz, ngraham, VDG

Reviewed By: ngraham, VDG

Subscribers: januz

Maniphest Tasks: T10330

Differential Revision: https://phabricator.kde.org/D18679