Glade support (mostly)

This adds proper Glade support for most libhandy widgets and fixes a few bugs along the way.

More specifically:

General fixes:

• HdyActionRow
  • Implement remove()
  • Fix an unrelated bug that allowed adding null prefixes
• HdyExpanderRow
  • Implement remove()
  • Fix forall() so that it actually iterates children you add
• HdyPreferencesWindow
  • Implement remove() and forall()
• HdyPreferencesPage
  • Implement remove() and forall()
  • Subclass GtkBin instead of GtkScrolledWindow
• HdyPreferencesGroup
  • Implement remove() and forall()
  • Subclass GtkBin instead of GtkBox
• HdyStackableBox
  • Don't hide last visible child when unfolded, this obviously causes children to disappear :x
  • Unset last visible child after removing or hiding, this prevents some interesting crashes when removing multiple children in a row
  • Disconnect notify::visible signal handler after removing a child
• HdyColumn
  • Queue resize after changing maximum width
• HdyViewSwitcher
  • Subclass GtkBin instead of GtkBox, this fixes a crash in Glade

Glade:

• Update the dtd, there are a bunch of elements that we didn't use previously
• Add GladeHdyUtils to at least slightly reduce code duplication
• HdyViewSwitcherTitle
  • Use proper display names for the policy enum
• HdyCarousel
  • Use GladeHdyUtils
  • Add a synthetic position child property for reordering
• HdyPreferencesGroup
  • Mark propeties as translatable
• HdyHeaderBar
  • Copy GtkHeaderBar adaptor
• HdySearchBar
  • Copy GtkSearchBar adaptor
• HdyColumn
  • Use saner defaults, see #98 (closed)
• HdyLeaflet/HdyDeck
  • Add an adaptor based on GtkStack.
    • Add a synthetic position child property for reordering
• HdyPreferencesWindow
  • Add an adaptor. Since we can't use placeholders, use an Add page action + create 3 pages by default + a synthetic position child property
• HdyPreferencesPage
  • Same as HdyPreferencesWindow, except page -> group
• HdyExpanderRow
  • Add an adaptor based on GtkListBox. Have a position child prop too.