Change Web interface to fork by default

Implemented to match the daemon's default action and option to not fork/daemonize.
move and update createicons & check_glade scripts
Create new generate_pot.py script for translations

 * glade3 files require workaround with intltool-extract
 * webui javascript files are now included
 * fix multiline string for parse
Replace Markup with gtkbuilder atttributes for simpler translation labels
#2218 #2254 : Re-enable utpex control and add lt_tex (tracker exchange)
Reword gtk prefs tooltip
Fix #2277 : Plugins keys weren't fetched in filter_torrent_ids

Fixed bug introduced in 8c106ce8c4c0794ddd63e8e8f98b097221a56a52
where keys for plugins weren't fetched in filter_torrent_ids.
Fix tracker_error_filter typo and tidyup commenting in filtermanager
Speed optimizations to sessionproxy.