Elipsize view source title to prevent toolbar overflow, fixes #4755

This also fixes on of the dot points in #4397

> ## Browse show source - close is off RHS of toolbar 
> XO-4 Sugar 0.100 13.4.0 build 1 Browse156
> The close button does not fit on the toolbar of show source, document source and activity source but toolkit source is ok

http://bugs.sugarlabs.org/ticket/4755