Breaking changes in Thunar Extensions API

[André Miranda]

Besides the pending issues [1] we need to solve before a Thunar dev 
release, I
wanted to handle as much deprecations as possible. At merge time, there were
1336 warnings, now they are "just" 691 [2].

A considerable amount of those deprecations are due GtkAction and 
GtkUIManager,
their replacements (GAction and GBuilder) aren't actually drop-ins [3], 
we would
need to re-implement the menu related code: use glade-based xmls for 
GMenu and
custom functions to achieve the merge capabilities of GtkUIManager and 
perhaps a
few more hacks I can't remember or predict. That was the approach adopted by
Nautilus, but its UI has been revamped since then, and I can't say how 
well it
would work for Thunar's "more traditional" UI.

It's possible to just live with the deprecation warnings (or suppress them)
until gtk4+, annoys me but I'm fine with that. Nevertheless, I think these
deprecations must be removed from Thunar Extensions API (thunarx) and 
the soname
version was already bumped to 3 in order to avoid conflicts with gtk2 
symbols.

This leads to the approach adopted for Nemo: clean the API and let all 
deprecated
stuff internally. I have already prepared a WIP branch[4] replacing the 
GtkAction
usage but I'd like to listen from you guys which way we should pick:

1) Keep GtkAction in thunarx, anyway we'll have to bump thunarx version 
again
    when porting to gtk4, hopefully there will be a better replacement for
    GtkAction and GtkUIManager by the time.

2) Just remove GtkAction from thunarx API. Hopefully by the time of gtk4 
port
    the soname version bump will be just to avoid symbols conflicts (my
    preference).

3) Clean all deprecated stuff from Thunar. (don't expect me to work on 
this).

Regarding the implementation of the WIP branch, I picked ThunarxMenuItem 
as the
replacement of GtkAction based on Nemo and Nautilus, but I'm open for
suggestions of better names. Also I didn't rename methods such as
get_file_actions, get_folder_actions and get_dnd_actions, so it's still 
a bit
awkward.

This change touches ThunarxMenuProviders and ThunarxPreferencesProviders 
only,
the other providers use API specific classes instead of GtkAction, 
that's the
same rationale for ThunarxMenuItem.

If you reached this point, thanks your patience and sorry for my limited 
skills
with words :). If you want to put your 2 cents or find something confusing,
please reply this e-mail or let's discuss this on #xfce-dev.

1 - https://wiki.xfce.org/releng/4.14/roadmap/thunar
2 - https://wiki.xfce.org/releng/4.14/roadmap/thunar/deprecations
3 - https://wiki.gnome.org/Projects/GTK+/Menus#Comments
4 - https://github.com/andreldm/thunar/tree/gtkaction-replacement

Cheers,
Andre Miranda (not Thunar's maintainer)