Add ability to select multiple conversations

[ianmacd]

Describe the bug

In the left pane, it's not possible to use Shift + Mouse 1 to select multiple conversations. The action selects only the conversation at the current cursor position.

This is desirable, for example, if one wishes to mark as read with a single action all messages in multiple groups.

To Reproduce

1. Select a conversation in the left pane by clicking on it.
2. Select a second conversation further down the left pane with Shift + Mouse 1.
3. Only the second conversation is selected, rather than both endpoints and all intervening conversations.

Other information (please complete the following information):

• Session version: 1.8.5.

[gnrlus]

It seems like Signal used to put radio buttons by each message and conversation and you could just click and select all you wanted. Then you could delete, forward, copy, that selection. For some reason they eliminated that feature and now it's much harder to use. Or if no radio buttons then click adjacent left/right of the message/conversation to select and Ctrl click to multiple select. Shift key to select all messages/conversations between two selections would be nice.