Remove the ThumbnailGenerator class

The only use of it is commented out. ThumbnailGenerator used the UNO-based
XToolkitExperimental stuff that I want to get rid of. If/when we want
thumbnails, we should use the existing thumbnails from document formats that
have them, or generate them using LOKit.

Also remove stuff from the Bootstrap class that was only used by
ThumbnailGenerator.

Conflicts:
	android/experimental/LOAndroid3/src/java/org/libreoffice/ui/LibreOfficeUIActivity.java
Reviewed on:
	https://gerrit.libreoffice.org/13503

Change-Id: Ia3a1a7f372a814359c5b496cdb17c35246e34817