message list improvements to reduce repetition

don't repeat the sender/avatar when it's the same for two consecutive messages

then also skip the context for threaded messages, when it's the same for two consecutive messages. to still visually distinguish threaded messages from normal messages, give them an alternative background

before:

after:

note that the latest version of the patch fixes the ugly 1px or 2px hidpi gaps by adjusting the full rect by -1, -1 also in the x1/y1 direction.