add initial search in shared files

@MTGap @icewind1991 customers start to ask for this. Not the best way to implement it because I use the folder backend get the children ids and then use those in a `IN (?,...,?)` SQL statement. This limits this implementation to the supported maximum length for a sql query ... which will be database specific. But it is a first implementation. I don't understand the sharing code well enough to implement a JOIN based implementation (actually that currently won't be possible because we have to recursively collect the children in a folder before we can filter their filename). 

@karlitschek @DeepDiver1975 @bartv2 please review.