Paragraph & document selection fixes

Fixes for various selection-related paper-cuts. Impacted commands are:

Move/extend to paragraph start - Ctrl(+Shift)+Up (issue #65)
Move/extend to paragraph end - Ctrl(+Shift)+Down (issue #66)
Move/extend to document start - Ctrl(+Shift)+Home
Move/extend to document end - Ctrl(+Shift)+End
Select-all - Ctrl+a

Additionally, there is a large performance boost especially for moving to the document start or end using the ctrl+home/end shortcuts, as the new implementation avoids costly `convertForwardStepsBetweenFilters` calls