add form field for bg image url default value

lang/cs.php

@@ -80,6 +80,7 @@ return [
 	'term.colors_preview' => '',
 	'term.debugbar' => 'Rozšířené ladění',
 	'term.ascii_debug' => 'Ladění vstupních dat',
+	'term.backdrop' => 'URL obrázku na pozadí',
 
 	'cursor.block_blink' => 'Blok, blikající',
     'cursor.block_steady' => 'Blok, stálý',

lang/en.php

@@ -79,6 +79,7 @@ return [
 	'term.colors_preview' => '',
 	'term.debugbar' => 'Debug internal state',
 	'term.ascii_debug' => 'Display control codes',
+	'term.backdrop' => 'Background image URL',
 
 	'cursor.block_blink' => 'Block, blinking',
     'cursor.block_steady' => 'Block, steady',

pages/cfg_term.php

@@ -172,6 +172,11 @@
 		<input class="tiny" type="text" name="bm5" id="bm5" value="%h:bm5%">
 	</div>
 
+	<div class="Row">
+		<label for="backdrop"><?= tr('term.backdrop') ?></label>
+		<input type="text" name="backdrop" id="backdrop" value="%h:backdrop%" required>
+	</div>
+
 	<div class="Row checkbox" >
 		<label><?= tr('term.crlf_mode') ?></label><!--
 		--><span class="box" tabindex=0 role=checkbox></span>
@@ -282,7 +287,6 @@
 		<input type="hidden" id="ascii_debug" name="ascii_debug" value="%ascii_debug%">
 	</div>
 
-
 	<div class="Row checkbox" >
 		<label><?= tr('term.fn_alt_mode') ?></label><!--
 		--><span class="box" tabindex=0 role=checkbox></span>