Cleared search is redone after switching library

to reproduce:

1. open two libraries
2. search in first one
3. clear search in first
4. switch to second library
5. switch back to first

then in first library the search is restored.

info:
```
JabRef 5.0-dev--snapshot--2019-04-03--master--a273eb1fa
Mac OS X 10.14.3 x86_64 
Java 1.8.0_191
```