move 'midiSources' classvar from AbstractCVWidgetEditor to CVWidget

Signed-off-by: nuss <st9fan@gmail.com>
CVWidgetMS: avoid bloating wdgtControllersAndModels
Within a CVWidgetMS' wdgtControllersAndModels has to store the actions resp. cvSpec only once. Avoid duplicating them in any slot.

Signed-off-by: nuss <st9fan@gmail.com>
CVWidgetMSEditor: ensure intransparent background when opening a new editor and first tab gets focused

Signed-off-by: nuss <st9fan@gmail.com>