use non-breaking space for button with no description

to preserve button's size

reverses some inappropriate replacements of .html with .text.

closes #5113
handle space-only strings, not just zero-length
add utils.escape_html
handle empty string in ToggleButtonsWidget
Merge pull request #5114 from minrk/empty-button

use non-breaking space for button with no description
Add breakpoint handling magics
Adding bprm command to remove breakpoints
Convert lineno to int

Make sure linenumber is an int in IPyBreakPoint.
Also added docstrings to magic functions.
Adding regex breakpoints