signal_generator: bumped version, added changelog file (#279)

.catalog/changelog.md

@@ -0,0 +1,6 @@
+## 1.2
+ - Fixed compatibility with PWM HAL API
+## 1.1
+ - Removed call to legacy UI SDK API
+## 1.0
+ - Initial release

application.fam

@@ -6,7 +6,7 @@ App(
     requires=["gui"],
     stack_size=1 * 1024,
     fap_description="Control GPIO pins to generate digital signals",
-    fap_version="1.1",
+    fap_version="1.2",
     fap_icon="signal_gen_10px.png",
     fap_category="GPIO",
     fap_icon_assets="icons",