Move calc_thumb_rect to Utils

We'll need it in the ComposeWindow, too.
Move slice_pixbuf to Utils

Also needed in the ComposeWindow
compose-window: Add some margin-left to the image button

Also, set the size request. This will be important once the inline
media's thumbnail in the compose window gets sliced correctly.
ComposeTweetWindow: React to drop events

This enables the user to drop images into the compose window to upload
them as normal inline media.