python_payload: Rename "menu" to "category" in flow3r.toml

python_payload/apps/tiny_sampler/flow3r.toml

 [app]
 name = "tiny sampler"
-menu = "Music"
+category = "Music"
 
 [entry]
 class = "TinySampler"

python_payload/apps/updat3r/flow3r.toml

 [app]
 name = "Check for Updates"
-menu = "Hidden"
+category = "Hidden"
 wifi_preference = true
 
 [entry]

python_payload/apps/w1f1/flow3r.toml

 [app]
 name = "WiFi"
-menu = "Hidden"
+category = "Hidden"
 
 [entry]
 class = "WifiApp"

python_payload/apps/wurzelitzer/flow3r.toml

 [app]
 name = "Wurzelitzer"
-menu = "Media"
+category = "Media"
 wifi_preference = true
 
 [metadata]