Improvements to the Linear Meter widget

[abrahamwolk]

This pull request implements several improvements to the Linear Meter widget:

1. Perhaps most noticeably, it implements a new mode: the indicator can now be drawn as a bar in addition to the existing functionality to be drawn as a needle. The following screenshot shows the bar-mode and the needle-mode side by side:
2. With the goal of improving performance and responsiveness on multicore processors, the concurrency of the Linear Meter has been made more fine-grained. Previously, concurrency issues were avoided by both reading and modifying state only on the JavaFX Application thread using Platform.runLater(). This pull request replaces that strategy by instead relying on a ReentrantReadWriteLock, allowing for simultaneous read-accesses while write-operations are sequential.
3. The widget property "Limits from PV" has been generalized: In addition to allowing for the options "Limits from PV" and "No limits from PV", the property now also allows for the options "Min & max from PV" and "Alarm limits from PV", allowing the user more fine-grained control over this option. This functionality has been implemented in a manner such that OPIs saved using the previous version of the Linear Meter will still be loaded correctly. However, an OPI created using the new version of the Linear Meter implemented in this pull request should not be loaded using an older version of Phoebus.
4. It is now possible to reduce the width of the widget to 10 pixels at minimum instead of 25 pixels.

[kasemir]

I think we should increase the version number, since the behavior of "limits from PV" changes and there's the new bar mode

 1.0.0: ...based on 3rd party library
 2.0.0: Simple linear meter,....
 3.0.0: Meter and bar mode; more ‘LimitsFromPV’ options

[kasemir]

It is now possible to reduce the width of the widget to 10 pixels at minimum instead of 25 pixels.

Without the scale, is it possible to narrow it further, down to 3 or 1 pixel?

[abrahamwolk]

I think we should increase the version number, since the behavior of "limits from PV" changes and there's the new bar mode

I did increase the version number in the following comment: 729bfa8.

Without the scale, is it possible to narrow it further, down to 3 or 1 pixel?

The limit is currently set to 10: 67de29a. It could probably be lowered further, but I am not sure the logic works for 1 pixel or even 3 pixels. Would a lower limit than 10 be useful for you?

[georgweiss]

Found in several commits:

Math symbols (minus) could be surrounded by space for better readability.

[abrahamwolk]

Fixed: fb5ad09

[kasemir]

I meant to refer to the LinearMeterWidget.METER_VERSION:

/**
 * 1.0.0: Linear meter by Claudio Rosatti based on 3rd party library
 * 2.0.0: Simple linear meter, based on meter v 3.0.0, drawn in background
 */
public static Version METER_VERSION = new Version(2, 0, 0);

A new version 3.0.0 would allow the LinearMeterConfigurator or other tools that create displays to know if the bar mode and the additional limits-from-pv behavior is available.

As for minimum width, we do have some large and cryptic overview displays full of this:

They are of course only meaningful to those in the know, but they use down to 1-pixel wide bars to indicate information.

[abrahamwolk]

Seting the width or height to 1 pixel doesn't work (no bar is visible), but setting it to 2 pixels results in a 1-pixel wide bar, given that the color "Foreground Color" is set to transparent. I have therefore updated the minimum width & height to be 2 pixels.