main: fix indentation, no tabs
main: add generic locale initialization
main: no need to explicitly clear static variables
core: port to GtkApplication and use the new application menus

This patch evolves the `DhBase' into a fully featured new `DhApp', which
inherits from GtkApplication. The new `DhApp' will take care of every
`DhWindow' and `DhAssistant' associated to the application.

Application-specific actions are added to the new application menu:
 * New window
 * Preferences
 * About Devhelp
 * Quit

As part of this process of setting up the menu, the Preferences dialog
was modified not to be tied to any specific window.

https://bugzilla.gnome.org/show_bug.cgi?id=677927
main: new `--new-window' command line option

We also want to be able to create new windows from command line calls. This is
implicit if Devhelp is not already running, but needs a special treatment
otherwise.
main: document the argument in the search-related options
main: improve the order of options given in `--help'
libdevhelp: provide a dh_init() to initialize common configuration
libdevhelp: bump soname
build: require gio 2.32

For all the new GMenuModel stuff.
app: ensure all windows get destroyed when quitting
gtk-builder: require gtk+ 3.0

Also add copyright header to the UI builder file.
app: build application menu from GtkBuilder description
window: convert to GtkBuilder and GActions

Converted the menu and toolbars to be GtkBuilder-based instead of GtkUIManager
based, as well as to use GActions instead of GtkActions.

The generated window menu is then set with gtk_application_set_menubar(), which
will make it be displayed properly also in fallback mode.
build: require gtk+ 3.4

For all the new GtkApplicationWindow stuff
Updated POTFILES.in
Updated Spanish translation
Updated Lithuanian translation
Updated Norwegian bokmål translation
Updated Serbian translation