tdf#122366 Store search filter value in sessionStorage

sessionStorage seems to be reset, when we change the module context.
If this is not acceptable, we can try localStorage.

Change-Id: I5ec2ff526e48dd2c7d1889578e991833e43f76c6
Reviewed-on: https://gerrit.libreoffice.org/71688
Tested-by: Jenkins
Reviewed-by: Olivier Hallot <olivier.hallot@libreoffice.org>

help3xsl/help.js

@@ -81,6 +81,17 @@ function debounce(fn, wait) {
 }
 search.addEventListener('keyup', debounce(filter, 100));
 
+// Preserve search input value during the session
+search.value = sessionStorage.getItem('searchsave');
+
+if (search.value !== undefined) {
+    filter();
+}
+
+window.addEventListener('unload', function(event) {
+    sessionStorage.setItem('searchsave', search.value);
+});
+
 // copy pycode and bascode to clipboard on mouse click
 // Show border when copy is done
 divcopyable(document.getElementsByClassName("bascode"));