Simplify CSS rules for cursor and composer element

The cursor node is now completely hidden when not in use, so there is no
need to touch the height or width properties of the composer or cursor.

In addition, the new overlay caret no longer requires the composer to be
displayed as an inline-block in order to function.
Provide a better caret position fallback when no visible rects are found

In the specific situation where there are no visible rectangles nearby
from which to calculate the caret position and size, the cursor node itself
will be made visible with some hidden internal text as a substitute.

This ensures the caret is correctly aligned in empty paragraphs that are
not left-aligned, or in an area surround by non-visible rects such as near
multiple annotations.
Remove another unnecessary inline-block

The cursor element is now only visible when it contains either text
or the caret-size helper. In addition, there are no longer any fixed
widths being applied to the cursor element, so the inline-block is not
necessary.
Merge pull request #611 from peitschie/cursor-visible

Provide a better caret position fallback when no visible rects are found