Do not allow the compose window's slider between headers and body to hide widgets on either side.
Add a mnemonic to compose window's Subject line, allowing alt+s to bring focus there.
Make Up key bring focus to Subject line, if the cursor is on the first line of body textview.
Remove test_strftime() that was mistakenly included in previous commits.
Use the GtkPaned's position value to store compose_notebook_height, since setting this position is what we're using it for now. Since GtkPaned doesn't have any "resize done" signal available, we have to do it in the notebook's "size-allocate" callback.
Sync Spanish manual with 6fc1c5d1