"Move Selected Text" breaks relative links in the text moved.

[K. Bauer]

Zim version 0.59 (precompiled) on Windows.

=== Related Bugs ===

https://bugs.launchpad.net/zim/+bug/501496
    "Relative links broken after cut and pasted to different hierarchy level"
    If ZIM uses the same code for processing copy/paste text and "Move Selected Text"-text, my bug may be considered a duplicate.

=== How to reproduce ===

1. Write a page ":Parent", that contains a relative link, e.g. [[+Sub1:Page]].
2. Mark, rightclick and select "Move Selected Text" and move to "+Sub2".

=== Expected behaviour ===

After moving the text, it the link should have changed (ideally) to [[Sub1:Page]], as Sub1 is no longer a subpage but on the same hierachy level with the text containing the link.

=== Observed behaviour ===

The link stays unchanged, i.e. [[+Sub1:Page]]. Instead of :Parent:Sub1:Page thus a non-existing page :Parent:Sub2:Sub1:Page is referenced. If unnoticed, after several restructurings, the user may be left having a hard time finding out what page was originally meant, especially if he has in the meantime created a page corresponding to the broken link.

[Jaap Karssenberg (jaap.karssenberg)]

https://github.com/jaap-karssenberg/zim-desktop-wiki/issues/46