Only tick if the ticker is disabled

[djc]

Fix a regression from #417 to fix #457.

Of the 4 callers of BarState::tick(), changed three to no longer tick if the ticker is enabled:

• ProgressBar::inc()
• ProgressBar::set_position()
• ProgressBar::update()

The fourth is ProgressBar::tick(), which already only ticked if the ticker was disabled.

[djc]

Seems like the performance impact here is negligible (based on running cargo run --release --example fastbar), although it's a little ugly that we have to lock another mutex.