Rename app-id to io.github.Hexchat

RDNS syntax is recommended for applications and this matches the
existing flatpak. Note that the dbus service and icons still need
renaming though the former is an API breakage.

Closes #2104
Closes #2103

data/misc/io.github.Hexchat.ThemeManager.desktop.in

@@ -0,0 +1,7 @@
+[Desktop Entry]
+Name=HexChat Theme Manager
+Exec=thememan %f
+Icon=hexchat
+Terminal=false
+Type=Application
+MimeType=application/x-hct;