main-windows: Add selection mode button and few other enhancements

On touchscreen devices you must press your finger for some seconds and with a mouse you must right-click on an item to activate selection mode.

If you don't know these operations it is impossible to activate selection mode.

Now there is a selection mode button like in all other gnome apps. If you click it, you go into selection mode and can select items with just one click.

To complete this mode of interaction, the auto-deactivation of the selection mode, when the last element gets deselected, and the visibility of the checkboxes in the contact list on hovering was removed.Kooha-2021-08-16-09_59_09