xtext: Fix overflow on long lines

xtext: Fix overflow on long lines

xtext keeps a static buffer and uses it for various things
and asserts that every text entry is < 4096. It does
this check on gtk_xtext_append*() except it does the check only on
the right half of text when indent is enabled.

This overflow caused corruption in the xtext struct
changing the url check functions making hovering
with the mouse do 'undefined' things.

In the long term this should be removed for a dynamically
allocated buffer so no arbitrary size limit exists and
text gets cut off.

Fixes #1465
Fixes #1186
Fixes #1206