toolbar: Simplify update

- Use set_visible() to show/hide widgets
- Call set_custom_title() only once
- Hide back button if selection mode is enabled
Lower required version of GTK+
Use libgd widgets instead of gtk+ if the later is too old

Import Stack, StackTransitionType and StackSwitcher from libgd if gtk+ is older
than 3.9.
Use HeaderBar from libgd if not available in gtk+

Also, rename data/Headerbar.ui to data/headerbar.ui.in which configure will use
as the template for data/headerbar.ui file.

Create po/POTFILES.skip and add data/headerbar.ui to it.
toolbar: Don't set custom title on Toolbar init

This makes sure that the label created by the header is realized on init.
The setting of the state of the toolbar to 0 will do this anyway.
window: Pack header bar if GTK+ version is older than 3.9

gtk_window_set_titlebar was introduced in 3.9.
toolbar: Destroy the window on close if GTK+ version is older than 3.9

gtk_window_close was introduced in 3.9.
window: Hide titlebar when maximized if GTK+ version is older than 3.9

We can't set a custom titlebar before 3.9, so hide the titlebar from the window
manager when maximized.
toolbar: Hide close button if not maximized and header bar is not title bar

If GTK+ version is older than 3.9 which allows custom title bars, show the
close button of the header bar if we are maximized and selection mode is not
enabled.
grilo: Add missing parameters to _on_source_removed
Simplify a bit _make_icon_frame

Have _draw_rounded_path hide away all the cairo stuff
Add simple cache to avoid redrawing cairo stuff
Simply copy the pixbuf

Instead of recreating it from the surface
Lock the dict while searching in frame_cache
fix indentation
Merge pull request #139 from gquintard/WIP/albumcache

Cache the pixbuf containing the rounded rectangle
build: fix subdir-objects warnings in automake 1.14
Merge pull request #155 from lubosz/master

build: fix subdir-objects warnings in automake 1.14
Merge pull request #128 from kyoushuu/gtk38-port

Make GNOME Music usable with GTK+ 3.8
add about dialog