slide-sorter: re-factor to improve bbox calcuation

Always use a SlsPageDescriptor to calculate bbox, so we can
adapt it to merge slides attributes where possible.

Change-Id: Id71945bbbb9adb069654ab712e22ea79fea96c1f
slidesorter - cleanup redundant caches and notifications.

Change-Id: Ibe8106a77899f141d1e3722bcae615ab905b2568
fix build after rebase

Conflicts:
	sd/source/ui/slidesorter/view/SlideSorterView.cxx
	sd/source/ui/slidesorter/view/SlsPageObjectLayouter.cxx
	sd/source/ui/slidesorter/view/SlsPageObjectPainter.cxx

Change-Id: Id014a49dbbb33123d2d04f9f33954b77575f2e9e
remove whitespaces

Change-Id: I9daea42a433b5032931a722878874917cf37f4d1
remove whitespace

Change-Id: Ib15413e73409cc33de01fa92a47b9d1237cfc4b2
remove whitespaces

Change-Id: Ie14ba3dcb97f20479a04538748ef2c1c9e6c5dac