minimap: Do not force highlighting updates if the document is very large
minimap: Do not force highlighting updates if the document is very large

If the document is larger than 7500 lines, don't force highlighting updates
when redrawing the minimap as this is potentially very slow.
For an example, a 13k lines python file needs an additional 1.3 seconds
to open on my notebook if the whole highlighting is being updated.

hlamer/kate,DickJ/kate,hlamer/kate,DickJ/kate,jfmcarreira/kate,jfmcarreira/kate,cmacq2/kate,sandsmark/kate,sandsmark/kate,sandsmark/kate,hlamer/kate,cmacq2/kate,DickJ/kate,DickJ/kate,hlamer/kate,hlamer/kate,jfmcarreira/kate,hlamer/kate,hlamer/kate,hlamer/kate,hlamer/kate,cmacq2/kate,hlamer/kate,sandsmark/kate