Basic search functionality on text items.
 - Hacked in UI elements, final UI depends on GTK3 progress
 - Can select text then hit Ctrl+F to search
 - F3 triggers "Find Next"
Refactoring and comments; made search loop over all pages
Merge branch 'gtk-builder' into search

Conflicts:
	src/xo-callbacks.c
	src/xo-callbacks.h
	src/xo-interface.c
Finishing merge with gtk-builder; re-added UI elements and hooked up callbacks
Added backwards searching
Fixed bug where search wouldn't wrap all the way around; refactoring and clean up
Merge branch 'gtk-builder' into search
Scrolling to items seems to work
Hooked up find dialog; fixed bug with find previous when using single-character strings
Rare Segfault Bugfix

Preconditions
 - "Button 3 Mapping" set to "Select Region".
 - "Hand Tool" selected.

Steps to reproduce:
 1. Right click (button 3) and hold, dragging the select region off the canvas.
 2. Without re-entering the canvas, "File"->"Annotate PDF", and select a
PDF to be the new background.
 3. Re-enter the canvas with the mouse pointer
 4. Segfault
Implemented case insensitive searching and added checkbox to find dialog for toggling
Items now sorted top-to-bottom within layers
Added popup when no matches are found; pages now remember their current layer instead of defaulting to the top layer
Implemented PDF search; added radio buttons to toggle search between current anotation layer and PDF background
Moved search functions into xo-search.c; made find dialog reappear in last position instead of recentered; added padding to find dialog; added docs
Merge branch 'gtk-builder'
Merge branch 'bookmarks'
Merge remote-tracking branch 'origin/search'

Conflicts:
	src/xo-callbacks.c
	src/xo-paint.c
	src/xournal.h
Renamed duplicate Glade file id's resulting from merge