Put blocks in "shortcut" category to put them at top of the block picker

Bug 1390339: Before, we had hard-coded the "image" and "text" blocks into
the top of the block picker. With this change, a block developer can
just give their blocktype the category "shortcut" and it will show up there.

Change-Id: Ibaafc481745dd290442aa125b637394084705b00
Merge "Put blocks in "shortcut" category to put them at top of the block picker"