Library panels: a persistent panel whose file is gone stays listed as missing

At startup, a persistent swatch or graphic style library panel whose
file could not be read was closed and left out of the preferences,
with only a status-bar note. The next preferences save dropped it, so
moving or renaming the file once lost the panel and its settings.

Such a panel now reopens where the saved layout had it and stays in
the preferences. Its tab shows the file's name without the extension,
and its body shows "Missing:" with the file's path and a Locate
Library… button. The new command ui.libraryPanel.locate {panel,
path?} loads the library from the file given, or from one picked in
an open dialog, and shows it in the same panel, which keeps its view,
Show Find Field and Persistent. When the file's library is open in
another panel, the missing panel closes and that panel is brought
forward and made persistent.

A persistent panel whose User Defined library file is deleted while
the app runs shows the library as missing the same way; other panels
still close. window.swatchLibrary and window.graphicStyleLibrary take
a missing library's id, so a closed missing panel opens again.
Previous Library and Next Library are dimmed in a missing panel's
menu.

Docs: docs/mcp.md, docs/ui-parity.md, docs/target-app-parity.md.

docs/mcp.md

@@ -1574,11 +1574,21 @@ of its kind in that panel (what Previous Library and Next Library do; a library
 brought forward instead), `view` is `smallThumb`, `mediumThumb`, `largeThumb`, `smallList` or `largeList`,
 `showFind` shows or hides the find field, and `persistent: true` is the panel menu's Persistent: the panel
 reopens when the app starts, with its view and find field, where the saved layout had it (or floating). A
-library loaded from a file is read again from its path; one whose file is gone is left out, and the status bar
-says which. A closed persistent panel can still be set, so `persistent: false` stops it reopening. The reply
-is `{panel, kind, library, view, showFind, persistent, open}`, and `ui.inspect` lists every panel's settings
+library loaded from a file is read again from its path. When the file is gone, the panel still reopens and
+stays in the preferences: its tab shows the file's name, its body shows `Missing:` with the file's path and a
+Locate Library… button, and the status bar names the file. A persistent panel whose User Defined library's
+file is deleted while the app runs stays open with the same tab and `Missing:` body; any other panel closes.
+A closed persistent panel can still be set, so `persistent: false` stops it reopening. The reply is
+`{panel, kind, library, view, showFind, persistent, open}`, and `ui.inspect` lists every panel's settings
 under `ui.library_panels`.
 
+`ui.libraryPanel.locate {panel, path?}` is Locate Library…: it loads the library at `path` (without one, it
+asks for a file) as Other Library… does and shows it in the open panel `panel`, which keeps its view, find
+field and Persistent, and answers as `ui.libraryPanel` does. When the file's library already has a panel open,
+`panel` closes and is forgotten, and that panel is brought forward, made persistent when `panel` was.
+`window.swatchLibrary` and `window.graphicStyleLibrary` also take a missing library's id, so a closed missing
+panel opens again; they answer its file's name without the extension and `count: 0`.
+
 ```json
 {"name":"invoke_menu","arguments":{"command":"window.swatchLibrary","params":{"library":"pastels"}}}
 {"name":"invoke_menu","arguments":{"command":"ui.libraryPanel","params":{"panel":"library:swatches:pastels","view":"smallList","persistent":true}}}