Make dropzone text translateable

Also make the text sticky for browsers that support it, ensuring that
it is always visible even after scrolling

core/language/en-GB/Misc.multids

@@ -8,6 +8,7 @@ ConfirmCancelTiddler: Do you wish to discard changes to the tiddler "<$text text
 ConfirmDeleteTiddler: Do you wish to delete the tiddler "<$text text=<<title>>/>"?
 ConfirmOverwriteTiddler: Do you wish to overwrite the tiddler "<$text text=<<title>>/>"?
 ConfirmEditShadowTiddler: You are about to edit a ShadowTiddler. Any changes will override the default system making future upgrades non-trivial. Are you sure you want to edit "<$text text=<<title>>/>"?
+DropMessage: Drop here (or click escape to cancel)
 InvalidFieldName: Illegal characters in field name "<$text text=<<fieldName>>/>". Fields can only contain lowercase letters, digits and the characters underscore (`_`), hyphen (`-`) and period (`.`)
 MissingTiddler/Hint: Missing tiddler "<$text text=<<currentTiddler>>/>" - click {{$:/core/images/edit-button}} to create
 RecentChanges/DateFormat: DDth MMM YYYY

themes/tiddlywiki/vanilla/base.tid

@@ -226,12 +226,17 @@ a.tw-tiddlylink-external:hover {
 	z-index: 10000;
 	display: block;
 	position: absolute;
+	position: -webkit-sticky;
+	position: -moz-sticky;
+	position: -o-sticky;
+	position: -ms-sticky;
+	position: sticky;
 	top: 0;
 	left: 0;
 	right: 0;
 	background: <<colour dropzone-background>>;
 	text-align: center;
-	content: "Drop here";
+	content: "<<lingo DropMessage>>";
 }
 
 /*