PVTree handling of missing PVs and 'expand errors'

[kasemir]

The PV tree shows when a PV is in alarm, but it doesn't specifically indicate missing PVs.

In this screenshot, the highlighted INPE has a missing PV. It is shown just like the also highlighted INPH, which has a constant value:

Expected: Missing PVs should show invalid/disconnected alarm.

Especially for large trees, the 'expand only errors' button in the toolbar should expand the tree so that nodes in alarm are shown. It currently seems to stop at the top node in alarm:

Expected (or at least more practical): Expand paths to all nodes in alarm.

[kasemir]

Update:

• Highlights disconnected PVs,
• Identifies constant links
• 'Expand tree branches in alarm' now works

[kasemir]

While #2067 fixes missing PVs and 'expand errors', noticed that the code could do with an overall rework:

• Separate model from (SWT) view
• Add JavaFX version of view
• Add standalone application based on JavaFX
• Add option to read fields as "field$", i.e. long string, to avoid the errors shown in screenshot where a long link text runs in the DBR_STRING length limitation