attachmentsPanel moved to MediaContainer
panel activation improved
minor fixes
attacher processing is added; panel initialization is fixed
filepicker downloading is on panel initialization now; post button enabling replaced
cr fixes are added & some code polishing is done
attachmentsPanel is view since now;
some fixes
cr fixes
Merge branch 'mikhailov/attachmentsPanel' into rc2.0.0

Conflicts:
	sdk-derived/controls/card-composer.js
	sdk-derived/plugins/cards/comment-card.js
	sdk-derived/plugins/cards/photo-card.js