Update link to GTK+2 bundle for test builds to self-hosted version

Make us more independent from ftp.gnome.org and
host the, probably never ever changing, bundle ourselves.

Say hello to Geany 1.35 "Siralt"!

Fix document_account_for_unsaved() so it does not alter the `changed`
flag on documents, in order for plugins to have a reliable value there
at all times.

Patch by @chrontec with small additional tidying up.

Closes #1857.

Autotools: Detect GTK version to use automatically.  GTK2 is still the
default if both GTK2 and GTK3 are present.

* Update zh_CN.po

* Update version number for zh_CN translation

* updated it.po

* Update some copyright times on it.po

This is the default since forever, so don't change it on the users,
no matter what is the new default in Scintilla 3.10.0.

Fixes #2010.

After 3b1c6f36 (PR#2005) a string
slightly changed, so update the translation to match.

* Update of Traditional Chinese translation

* Correct version number on zh_TW.po

Improve goto-symbols popup

- Notably the utils_strv_{find_common_prefix,find_lcs,shorten_file_list}
  now take -1 for num to mean to compute the array length.

- utils_strv_find_common_prefix implementation simplified.

- if num == 0 is passed to the above functions the passed strv is not
  dereferenced (so could be NULL).

- Fix lots of compiler warnings
- Fix a bug where a long base name would prevent ellipsizing the longest
  common substring
- rewrite utils_strv_shorten_file_list to be more clear (hopefully)
- use g_strlcpy
- optimize case where the longest common substring need not be searched for

The array annotation has many possible parameters, this avoids having a Doxygen
command for each one.

Luckily you can define Doxygen commands multiple times with different a number
of parameters each.

Since I based the algorithm of the above function on code in one of my python
plugins, I would like to remove the implementation in my plugin and call
Geany's function.

We can now use @array and @arraylen{param} annotations for arrays that
will make it to the generated gtkdoc header.

g-ir-scanner cannot properly parse 'gchar **' parameters without this.