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
Added fixes for handling of files with accents and other non-alphanumeric characters for Windows
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
Begin stage of adding gradient functionality to xournal
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
Architectural changes to allow for strokes to contain more than one color
Removed files accidently added previously
Removed another file
Some changes to get random gradient implemented, still needs work
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