Improve message selection

[jlo62]

Selecting a message, e.g. pressing on the same height next to it, has some flaws:

• what's even the point of being able to select them?\ The menu it opens: the top button is for deselecting them all, and the other 3 have nothing to do with selecting the messages.\ These are literally the only things I can do with selecting them.\ the other 3 should rather always be visible
• pressing the field for selecting it a second time should at least deselect it

[SilverRainZ]

Yes, They are not well-thought-out interactions.

• pressing the field for selecting it a second time should at least deselect it

  This is why the first button exists, it seems not very easy to implement, but I forgot the reason :'(

• The 2nd and 3rd buttons should always be visible… but I think always being visible is annoying, they may cover my messages.
• The last button should only show when the message list doesn't reach the bottom

Same to #412, I will try to fix it, but not very soon, I am no longer active on IRC and can not be a good maintainer now :'( If anyone wants to help, feel free to file PR.