docs(bal): rewrite CHECK ALIVE help text (DATA/RAW modes) + fix two typos (v0.3.8)

Rewrite the CHECK ALIVE help popup (ThresholdTimeWidget.help_text) to explain
both modes, keeping the <b>CHECK ALIVE</b> title bold:
- DATA mode: set the check-alive date; on wallet open, if the date has passed
  the plugin asks whether to postpone the inheritance.
- RAW mode: when less than this time is missing, ask to invalidate; failing to
  invalidate delivers the transactions to the heirs; kept the d/y suffix help.

Fix two typos within this help text: 'less then' -> 'less than',
'currrent' -> 'current'.

Bump version to 0.3.8 across the four version files. CHANGELOG entry #14.

bal/manifest.json

@@ -1,7 +1,7 @@
 {
     "name": "bal",
     "fullname": "Bitcoin After Life",
-    "version": "0.3.7",
+    "version": "0.3.8",
     "description": "Provides free and decentralized Bitcoin inheritance support. Build time-locked 'will' transactions that transfer funds to your heirs if you stop refreshing them (dead-man's switch), optionally relayed by will-executor servers.",
     "author": "Svatantrya",
     "licence": "MIT",