Gallery view: image editing toolbar

After #10 the user will be able to walk through the images.

There also must be a toolbar with image editing operations:
* Rotate
* Brush
* Blur
* Crop
* Text

Each of operations switches image mode from _viewing_ to _editing_. Each operation has the same logic of cancelling the operation, i.e. leaving the _editing_ mode without applying the operation. We could implement it in this scope:
* mark the _editing_ mode when any operation is clicked
* remove _editing_ marks when "cancel" button is clicked

Discuss with Sona:
* should we display "cancel" button when we activate any of the operations?
* should it be possible to hide the toolbar?