Make the monitor window's divider draggable and remembered (#144) (#145)

The cluster and QSO monitor split its two tables at half the height, fixed:
the Compose port had left the JavaFX SplitPane out until a splitter existed.
SplitterPane exists now, so the monitor uses it and reads and writes the same
profile value the SplitPane did (GUIpnl_directedMSGWin_dividerpositionDefault,
default 0.8), so a profile gets its old position back. Each heading now sits
in the pane of its table, and the "DX cluster" heading is translated
(monitor.dxCluster).

Fixes #144.


Claude-Session: https://claude.ai/code/session_01AaorTymXfkXPm8yb4Btva7

Co-authored-by: Claude <noreply@anthropic.com>

app-desktop/src/main/i18n/strings_en.properties

@@ -435,6 +435,7 @@ column.lastQrg=Last QRG
 column.airplane=AP [minutes / pot%]
 column.score=Score
 monitor.title=Cluster & QSO of the other
+monitor.dxCluster=DX cluster
 profiles.createFailed=The profile could not be created. The profile registry could not be written.
 profiles.createdWithoutCredentials=The profile "{0}" was created without callsign and password. Enter them on the Station tab after switching to it.
 profiles.duplicateFailed=The profile could not be duplicated.