Moving to pager previous page
Added handler
Added to json
Moved general methods to general data source class
Generalized viewer data sources
refs #855
Adjusted viewer library photo tile style
refs #846
Merge branch 'master' into pager-back

Conflicts:
	pisak/res/json/symboler/main.json
Improved and condensed data source getters and indexing
Refactored pager methods for next initial and previous page
Added general method for introducing new page
refs #855
Added doc for pager methods
Moved if clauses between pager methods
refs #855
Merge branch 'master' into pager-back