Refresh browser

Do not refresh during loading. Fix a problem discovered by looking at https://trac.openmicroscopy.org.uk/ome/ticket/11106

To Test:
 * Browse a dataset
 * Make sure the Refresh button is disabled when the thumbnails are loaded.
 * Click refresh. 
 * Button should be greyed out during the loading
* Go to the table view
* Go back to thumbnails view. The thumbnails should not be loaded.

Stop insight 
 * Open the file ```config>databrowser.xml```
* Edit the following entry
```
<entry name="/views/MAX_ENTRIES" type="integer">100</entry>
```
Replace 100 by 10 for example

 * Start insight
 * Browse a dataset with more than the value entered (e.g. 11)
 * The table view should be shown by default.
 * Go the thumbnails view.
 * The thumbnails should be loaded.