Add an action panel button to mark all conversations as read.

[ianmacd]

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• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest clearnet branch
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By pressing the ✓ button in the action panel, the user can now mark all conversations as read.

This PR incidentally implements tool-tips for the action panel buttons, as pictured here:

On successful marking, a toast is displayed:

Fixes #2346.

[ianmacd]

I've now implemented a modal dialogue to allow the user to confirm the marking of all conversations as read.

[KeeJef]

Generally think having a button to mark all conversations as read is a good idea, but not sure about the placement of the button in this design, the UX/UI team is going to look at some options on where we could place the button

[ianmacd]

Generally think having a button to mark all conversations as read is a good idea, but not sure about the placement of the button in this design, the UX/UI team is going to look at some options on where we could place the button

I'm not married to the location I used. I placed the button in the action panel because:

• Other such buttons already existed, so I could see at a glance how to add another;
• The action panel is chronically underutilised. There's a lot of unused vertical space, so it seems prudent to find new purposes for it.

[KeeJef]

Wanted to do a temperature check on this one to see if people still feel this is a necessary feature now that we are syncing the read status of messages. For me, this has really cut down on the need to go through conversations and mark them as read on desktop.