It's a horrible idea to have global VCL Images, since that will
inevitably crash on shutdown when the static dtor runs after
DeInitVCL, which breaks ~every JunitTest now.

0x00002af6750d2d51 in rtl::Reference<sfx2::sidebar::Theme>::~Reference (this=0x2af6756ceb28 <sfx2::sidebar::Theme::mpInstance>, __in_chrg=<optimized out>)

Try to fix that by moving the global sidebar::Theme instance to
SfxApplication where it can hopefully be deleted before shutdown.

Change-Id: Ia78f1e458699335b53a741b6463ce48af69584a7