qml dynamic status bar

[zoli]

• Responding to view.set_status, view.get_status, view.erase_status
• Responding to status_message
• Showing tab size
• Showing the display syntax

[bwiggs]

Some more info from @zoli in the gitter chat:

for each view status bar we store statuses in a map here https://github.com/limetext/lime/blob/master/backend/view.go#L47 currently we can modify them by SetStatus, GetStatus and EraseStatus there should be a connection between setting status and front end which is not implemented yet

[zoli]

Implemented in termbox frontend via #508, Most of the code should be the same for qml frontend to.

[erbridge]

Migrated to limetext/lime-qml#3.