Refactor "current" buffer handling

Fixes #42: BufferPage: Qt.application.active messes up highlight
Resolve #44: Enable automatic reconnect
Resolve #41: The color of side panel handles

Theme.secondaryColor doesn't make much difference compared to
Theme.primaryColor, so we'll use the same red shade highlight color as
we use for messages and Theme.highlightColor for the normal state. That
is the same color as pulley menus use.
Move busy (connecting) animation from pulleys to the side panel

It's a bit unintuitive to flash the pulleys when the real action is
actually going on in the side panel => move the busy flash there.
Tweak new message highlighting

Use bold instead of Theme.highlightColor which makes it look a bit too
colorful and even unreadable in the worst case when the highlight color
has a poor contrast over the background. Turn new unseen messages bold
instead, and do not highlight newly arrived messages while the view is
open/active/current/visible.

Ref: #40 - Unreadable highlight color for new messages