libshell: Scroll signals from LogWidget

Also it is now possible to hide the text-based scroll indicator.
Shell|GUI: Native scroll bar for the log entry history widget
Shell|GUI|libshell: LogWidget specifies page size depending on height rule

One "page up/down" is the height of the widget - 1.
Shell|GUI: Added mouse wheel scrolling and refactored console page

The console page is now managed by the ConsolePage widget, cleaning
up the main LinkWindow implementation somewhat. ConsolePage listens
to mouse wheel events and scrolls the log history accordingly.