Fix flickering on Instant Preview finishing original regeneration

The problem happened mipmap regeneration. Basically, we should not
start regenerating tile mipmaps until all the updates have been
completed. Otherwise, some parts of the regenerated mipmap will
have outdated (old) content and the user will see flickering.

Now KisImage has two special signals that notify the canvas thay
a special "lod rest updates batch" is coming. The canvas will block
tile regeneration for this batch and will start it right after the
batch is completed. Therefore the user will see completely prepared
data.

BUG:361448
Fixes T2145
CC:kimageshop@kde.org