MSB: Working on MATE 1.15 Development Cycle

Since Slackware 14.2 has been released and MATE 1.14 is out there for public, i can now focus on testing MATE 1.15 development cycle, aiming for MATE 1.16 which probably be around September/Oktober since it's aiming for Ubuntu 16.10 and Fedora 25 releases in October and November.

MSB master branch is now tracking MATE 1.15 cycle already and i have pushed several commits which upgrade to 1.15 and also enable more features in some components. Here's the highlight:

• Bump all packages to 1.15.x
• Add new deps: libgxps for atril to support XPS file format
• Add new flag for atril to support DVI format (it uses tetex's kpathsea)
• Add new deps: vte3 for mate-terminal (2.91 APIs)
• Add new deps for vte3: vala
• Engrampa, mate-notification-daemon, mate-polkit, mate-terminal and mate-session-manager are now GTK+3 only
• Enable introspection in caja and mate-desktop
• Enable gdict applet in mate-utils
• Enable IPv6, stickynotes, and polkit support in mate-applets

I have rebased GTK3 branch from latest master branch as well with some commits are removed since it's already being part of master branch changes.

Additional changes for full GTK+3 build:
* Add new deps: libunique3 (replacing libunique)
* Add new deps: glade
* Add new deps: gtksourceview3 (replacing gtksourceview)
* Removed deps: pygtksourceview
* Atril: Use webkit2gtk for ePUB support


Please note that this is a development cycle, so if you prefer stable releases, please use the 14.2-mate-1.14 branch. All binary packages for Slackware 14.2 will be built from this branch instead of master branch.

Feel free to play with 1.15 releases and let me know if there's a missing deps or any other suggestion for MATE Desktop packaging for Slackware.